This section presents the API reference for the ORM Query
object. For a walkthrough
of how to use this object, see Object Relational Tutorial.
Query
is produced in terms of a given Session
, using the Session.query()
method:
q = session.query(SomeMappedClass)
Following is the full interface for the Query
object.
Object Name | Description |
---|---|
ORM-level SQL construction object. |
sqlalchemy.orm.query.
Query
(entities, session=None)¶ORM-level SQL construction object.
Query
is the source of all SELECT statements generated by the
ORM, both those formulated by end-user query operations as well as by
high level internal operations such as related collection loading. It
features a generative interface whereby successive calls return a new
Query
object, a copy of the former with additional
criteria and options associated with it.
Query
objects are normally initially generated using the
Session.query()
method of Session
, and in
less common cases by instantiating the Query
directly and
associating with a Session
using the
Query.with_session()
method.
For a full walkthrough of Query
usage, see the
Object Relational Tutorial.
sqlalchemy.orm.query.Query.
__init__
(entities, session=None)¶Construct a Query
directly.
E.g.:
q = Query([User, Address], session=some_session)
The above is equivalent to:
q = some_session.query(User, Address)
sqlalchemy.orm.query.Query.
add_column
(column)¶Add a column expression to the list of result columns to be returned.
Pending deprecation: add_column()
will be superseded by
add_columns()
.
sqlalchemy.orm.query.Query.
add_columns
(*column)¶Add one or more column expressions to the list of result columns to be returned.
sqlalchemy.orm.query.Query.
add_entity
(entity, alias=None)¶add a mapped entity to the list of result columns to be returned.
sqlalchemy.orm.query.Query.
all
()¶Return the results represented by this Query
as a list.
This results in an execution of the underlying SQL statement.
Warning
The Query
object,
when asked to return either
a sequence or iterator that consists of full ORM-mapped entities,
will deduplicate entries based on primary key. See the FAQ for
more details.
sqlalchemy.orm.query.Query.
as_scalar
()¶Return the full SELECT statement represented by this
Query
, converted to a scalar subquery.
Analogous to SelectBase.as_scalar()
.
sqlalchemy.orm.query.Query.
autoflush
(setting)¶Return a Query with a specific ‘autoflush’ setting.
Note that a Session with autoflush=False will not autoflush, even if this flag is set to True at the Query level. Therefore this flag is usually used only to disable autoflush for a specific Query.
sqlalchemy.orm.query.Query.
column_descriptions
¶Return metadata about the columns which would be
returned by this Query
.
Format is a list of dictionaries:
user_alias = aliased(User, name='user2')
q = sess.query(User, User.id, user_alias)
# this expression:
q.column_descriptions
# would return:
[
{
'name':'User',
'type':User,
'aliased':False,
'expr':User,
'entity': User
},
{
'name':'id',
'type':Integer(),
'aliased':False,
'expr':User.id,
'entity': User
},
{
'name':'user2',
'type':User,
'aliased':True,
'expr':user_alias,
'entity': user_alias
}
]
sqlalchemy.orm.query.Query.
correlate
(*args)¶Return a Query
construct which will correlate the given
FROM clauses to that of an enclosing Query
or
select()
.
The method here accepts mapped classes, aliased()
constructs,
and mapper()
constructs as arguments, which are resolved into
expression constructs, in addition to appropriate expression
constructs.
The correlation arguments are ultimately passed to
Select.correlate()
after coercion to expression constructs.
The correlation arguments take effect in such cases
as when Query.from_self()
is used, or when
a subquery as returned by Query.subquery()
is
embedded in another select()
construct.
sqlalchemy.orm.query.Query.
count
()¶Return a count of rows this the SQL formed by this Query
would return.
This generates the SQL for this Query as follows:
SELECT count(1) AS count_1 FROM (
SELECT <rest of query follows...>
) AS anon_1
The above SQL returns a single row, which is the aggregate value
of the count function; the Query.count()
method then returns
that single integer value.
Warning
It is important to note that the value returned by
count() is not the same as the number of ORM objects that this
Query would return from a method such as the .all() method.
The Query
object,
when asked to return full entities,
will deduplicate entries based on primary key, meaning if the
same primary key value would appear in the results more than once,
only one object of that primary key would be present. This does
not apply to a query that is against individual columns.
For fine grained control over specific columns to count, to skip the
usage of a subquery or otherwise control of the FROM clause, or to use
other aggregate functions, use expression.func
expressions in conjunction with Session.query()
, i.e.:
from sqlalchemy import func
# count User records, without
# using a subquery.
session.query(func.count(User.id))
# return count of user "id" grouped
# by "name"
session.query(func.count(User.id)).\
group_by(User.name)
from sqlalchemy import distinct
# count distinct "name" values
session.query(func.count(distinct(User.name)))
sqlalchemy.orm.query.Query.
cte
(name=None, recursive=False)¶Return the full SELECT statement represented by this
Query
represented as a common table expression (CTE).
Parameters and usage are the same as those of the
SelectBase.cte()
method; see that method for
further details.
Here is the PostgreSQL WITH
RECURSIVE example.
Note that, in this example, the included_parts
cte and the
incl_alias
alias of it are Core selectables, which
means the columns are accessed via the .c.
attribute. The
parts_alias
object is an aliased()
instance of the
Part
entity, so column-mapped attributes are available
directly:
from sqlalchemy.orm import aliased
class Part(Base):
__tablename__ = 'part'
part = Column(String, primary_key=True)
sub_part = Column(String, primary_key=True)
quantity = Column(Integer)
included_parts = session.query(
Part.sub_part,
Part.part,
Part.quantity).\
filter(Part.part=="our part").\
cte(name="included_parts", recursive=True)
incl_alias = aliased(included_parts, name="pr")
parts_alias = aliased(Part, name="p")
included_parts = included_parts.union_all(
session.query(
parts_alias.sub_part,
parts_alias.part,
parts_alias.quantity).\
filter(parts_alias.part==incl_alias.c.sub_part)
)
q = session.query(
included_parts.c.sub_part,
func.sum(included_parts.c.quantity).
label('total_quantity')
).\
group_by(included_parts.c.sub_part)
See also
sqlalchemy.orm.query.Query.
delete
(synchronize_session='evaluate')¶Perform a bulk delete query.
Deletes rows matched by this query from the database.
E.g.:
sess.query(User).filter(User.age == 25).\
delete(synchronize_session=False)
sess.query(User).filter(User.age == 25).\
delete(synchronize_session='evaluate')
Warning
The Query.delete()
method is a “bulk” operation,
which bypasses ORM unit-of-work automation in favor of greater
performance. Please read all caveats and warnings below.
synchronize_session¶ –
chooses the strategy for the removal of matched objects from the session. Valid values are:
False
- don’t synchronize the session. This option is the most
efficient and is reliable once the session is expired, which
typically occurs after a commit(), or explicitly using
expire_all(). Before the expiration, objects may still remain in
the session which were in fact deleted which can lead to confusing
results if they are accessed via get() or already loaded
collections.
'fetch'
- performs a select query before the delete to find
objects that are matched by the delete query and need to be
removed from the session. Matched objects are removed from the
session.
'evaluate'
- Evaluate the query’s criteria in Python straight
on the objects in the session. If evaluation of the criteria isn’t
implemented, an error is raised.
The expression evaluator currently doesn’t account for differing string collations between the database and Python.
the count of rows matched as returned by the database’s “row count” feature.
Warning
Additional Caveats for bulk query deletes
This method does not work for joined
inheritance mappings, since the multiple table
deletes are not supported by SQL as well as that the
join condition of an inheritance mapper is not
automatically rendered. Care must be taken in any
multiple-table delete to first accommodate via some other means
how the related table will be deleted, as well as to
explicitly include the joining
condition between those tables, even in mappings where
this is normally automatic. E.g. if a class Engineer
subclasses Employee
, a DELETE against the Employee
table would look like:
session.query(Engineer).\
filter(Engineer.id == Employee.id).\
filter(Employee.name == 'dilbert').\
delete()
However the above SQL will not delete from the Engineer table, unless an ON DELETE CASCADE rule is established in the database to handle it.
Short story, do not use this method for joined inheritance mappings unless you have taken the additional steps to make this feasible.
The polymorphic identity WHERE criteria is not included for single- or joined- table updates - this must be added manually even for single table inheritance.
The method does not offer in-Python cascading of relationships - it is assumed that ON DELETE CASCADE/SET NULL/etc. is configured for any foreign key references which require it, otherwise the database may emit an integrity violation if foreign key references are being enforced.
After the DELETE, dependent objects in the
Session
which were impacted by an ON DELETE
may not contain the current state, or may have been
deleted. This issue is resolved once the
Session
is expired, which normally occurs upon
Session.commit()
or can be forced by using
Session.expire_all()
. Accessing an expired
object whose row has been deleted will invoke a SELECT
to locate the row; when the row is not found, an
ObjectDeletedError
is
raised.
The 'fetch'
strategy results in an additional
SELECT statement emitted and will significantly reduce
performance.
The 'evaluate'
strategy performs a scan of
all matching objects within the Session
; if the
contents of the Session
are expired, such as
via a proceeding Session.commit()
call, this will
result in SELECT queries emitted for every matching object.
The MapperEvents.before_delete()
and
MapperEvents.after_delete()
events are not invoked from this method. Instead, the
SessionEvents.after_bulk_delete()
method is provided to
act upon a mass DELETE of entity rows.
sqlalchemy.orm.query.Query.
distinct
(*expr)¶Apply a DISTINCT
to the query and return the newly resulting
Query
.
Note
The distinct()
call includes logic that will automatically
add columns from the ORDER BY of the query to the columns
clause of the SELECT statement, to satisfy the common need
of the database backend that ORDER BY columns be part of the
SELECT list when DISTINCT is used. These columns are not
added to the list of columns actually fetched by the
Query
, however, so would not affect results.
The columns are passed through when using the
Query.statement
accessor, however.
*expr¶ – optional column expressions. When present,
the PostgreSQL dialect will render a DISTINCT ON (<expressions>)
construct.
sqlalchemy.orm.query.Query.
enable_assertions
(value)¶Control whether assertions are generated.
When set to False, the returned Query will not assert its state before certain operations, including that LIMIT/OFFSET has not been applied when filter() is called, no criterion exists when get() is called, and no “from_statement()” exists when filter()/order_by()/group_by() etc. is called. This more permissive mode is used by custom Query subclasses to specify criterion or other modifiers outside of the usual usage patterns.
Care should be taken to ensure that the usage pattern is even possible. A statement applied by from_statement() will override any criterion set by filter() or order_by(), for example.
sqlalchemy.orm.query.Query.
enable_eagerloads
(value)¶Control whether or not eager joins and subqueries are rendered.
When set to False, the returned Query will not render
eager joins regardless of joinedload()
,
subqueryload()
options
or mapper-level lazy='joined'
/lazy='subquery'
configurations.
This is used primarily when nesting the Query’s
statement into a subquery or other
selectable, or when using Query.yield_per()
.
sqlalchemy.orm.query.Query.
except_
(*q)¶Produce an EXCEPT of this Query against one or more queries.
Works the same way as Query.union()
. See
that method for usage examples.
sqlalchemy.orm.query.Query.
except_all
(*q)¶Produce an EXCEPT ALL of this Query against one or more queries.
Works the same way as Query.union()
. See
that method for usage examples.
sqlalchemy.orm.query.Query.
execution_options
(**kwargs)¶Set non-SQL options which take effect during execution.
The options are the same as those accepted by
Connection.execution_options()
.
Note that the stream_results
execution option is enabled
automatically if the Query.yield_per()
method is used.
See also
sqlalchemy.orm.query.Query.
exists
()¶A convenience method that turns a query into an EXISTS subquery of the form EXISTS (SELECT 1 FROM … WHERE …).
e.g.:
q = session.query(User).filter(User.name == 'fred')
session.query(q.exists())
Producing SQL similar to:
SELECT EXISTS (
SELECT 1 FROM users WHERE users.name = :name_1
) AS anon_1
The EXISTS construct is usually used in the WHERE clause:
session.query(User.id).filter(q.exists()).scalar()
Note that some databases such as SQL Server don’t allow an
EXISTS expression to be present in the columns clause of a
SELECT. To select a simple boolean value based on the exists
as a WHERE, use literal()
:
from sqlalchemy import literal
session.query(literal(True)).filter(q.exists()).scalar()
sqlalchemy.orm.query.Query.
filter
(*criterion)¶Apply the given filtering criterion to a copy
of this Query
, using SQL expressions.
e.g.:
session.query(MyClass).filter(MyClass.name == 'some name')
Multiple criteria may be specified as comma separated; the effect
is that they will be joined together using the and_()
function:
session.query(MyClass).\
filter(MyClass.name == 'some name', MyClass.id > 5)
The criterion is any SQL expression object applicable to the
WHERE clause of a select. String expressions are coerced
into SQL expression constructs via the text()
construct.
See also
Query.filter_by()
- filter on keyword expressions.
sqlalchemy.orm.query.Query.
filter_by
(**kwargs)¶Apply the given filtering criterion to a copy
of this Query
, using keyword expressions.
e.g.:
session.query(MyClass).filter_by(name = 'some name')
Multiple criteria may be specified as comma separated; the effect
is that they will be joined together using the and_()
function:
session.query(MyClass).\
filter_by(name = 'some name', id = 5)
The keyword expressions are extracted from the primary
entity of the query, or the last entity that was the
target of a call to Query.join()
.
See also
Query.filter()
- filter on SQL expressions.
sqlalchemy.orm.query.Query.
first
()¶Return the first result of this Query
or
None if the result doesn’t contain any row.
first() applies a limit of one within the generated SQL, so that only one primary entity row is generated on the server side (note this may consist of multiple result rows if join-loaded collections are present).
Calling Query.first()
results in an execution of the underlying
query.
sqlalchemy.orm.query.Query.
from_self
(*entities)¶return a Query that selects from this Query’s SELECT statement.
Query.from_self()
essentially turns the SELECT statement
into a SELECT of itself. Given a query such as:
q = session.query(User).filter(User.name.like('e%'))
Given the Query.from_self()
version:
q = session.query(User).filter(User.name.like('e%')).from_self()
This query renders as:
SELECT anon_1.user_id AS anon_1_user_id,
anon_1.user_name AS anon_1_user_name
FROM (SELECT "user".id AS user_id, "user".name AS user_name
FROM "user"
WHERE "user".name LIKE :name_1) AS anon_1
There are lots of cases where Query.from_self()
may be useful.
A simple one is where above, we may want to apply a row LIMIT to
the set of user objects we query against, and then apply additional
joins against that row-limited set:
q = session.query(User).filter(User.name.like('e%')).\
limit(5).from_self().\
join(User.addresses).filter(Address.email.like('q%'))
The above query joins to the Address
entity but only against the
first five results of the User
query:
SELECT anon_1.user_id AS anon_1_user_id,
anon_1.user_name AS anon_1_user_name
FROM (SELECT "user".id AS user_id, "user".name AS user_name
FROM "user"
WHERE "user".name LIKE :name_1
LIMIT :param_1) AS anon_1
JOIN address ON anon_1.user_id = address.user_id
WHERE address.email LIKE :email_1
Automatic Aliasing
Another key behavior of Query.from_self()
is that it applies
automatic aliasing to the entities inside the subquery, when
they are referenced on the outside. Above, if we continue to
refer to the User
entity without any additional aliasing applied
to it, those references wil be in terms of the subquery:
q = session.query(User).filter(User.name.like('e%')).\
limit(5).from_self().\
join(User.addresses).filter(Address.email.like('q%')).\
order_by(User.name)
The ORDER BY against User.name
is aliased to be in terms of the
inner subquery:
SELECT anon_1.user_id AS anon_1_user_id,
anon_1.user_name AS anon_1_user_name
FROM (SELECT "user".id AS user_id, "user".name AS user_name
FROM "user"
WHERE "user".name LIKE :name_1
LIMIT :param_1) AS anon_1
JOIN address ON anon_1.user_id = address.user_id
WHERE address.email LIKE :email_1 ORDER BY anon_1.user_name
The automatic aliasing feature only works in a limited way,
for simple filters and orderings. More ambitious constructions
such as referring to the entity in joins should prefer to use
explicit subquery objects, typically making use of the
Query.subquery()
method to produce an explicit subquery object.
Always test the structure of queries by viewing the SQL to ensure
a particular structure does what’s expected!
Changing the Entities
Query.from_self()
also includes the ability to modify what
columns are being queried. In our example, we want User.id
to be queried by the inner query, so that we can join to the
Address
entity on the outside, but we only wanted the outer
query to return the Address.email
column:
q = session.query(User).filter(User.name.like('e%')).\
limit(5).from_self(Address.email).\
join(User.addresses).filter(Address.email.like('q%'))
yielding:
SELECT address.email AS address_email
FROM (SELECT "user".id AS user_id, "user".name AS user_name
FROM "user"
WHERE "user".name LIKE :name_1
LIMIT :param_1) AS anon_1
JOIN address ON anon_1.user_id = address.user_id
WHERE address.email LIKE :email_1
Looking out for Inner / Outer Columns
Keep in mind that when referring to columns that originate from
inside the subquery, we need to ensure they are present in the
columns clause of the subquery itself; this is an ordinary aspect of
SQL. For example, if we wanted to load from a joined entity inside
the subquery using contains_eager()
, we need to add those
columns. Below illustrates a join of Address
to User
,
then a subquery, and then we’d like contains_eager()
to access
the User
columns:
q = session.query(Address).join(Address.user).\
filter(User.name.like('e%'))
q = q.add_entity(User).from_self().\
options(contains_eager(Address.user))
We use Query.add_entity()
above before we call
Query.from_self()
so that the User
columns are present
in the inner subquery, so that they are available to the
contains_eager()
modifier we are using on the outside,
producing:
SELECT anon_1.address_id AS anon_1_address_id,
anon_1.address_email AS anon_1_address_email,
anon_1.address_user_id AS anon_1_address_user_id,
anon_1.user_id AS anon_1_user_id,
anon_1.user_name AS anon_1_user_name
FROM (
SELECT address.id AS address_id,
address.email AS address_email,
address.user_id AS address_user_id,
"user".id AS user_id,
"user".name AS user_name
FROM address JOIN "user" ON "user".id = address.user_id
WHERE "user".name LIKE :name_1) AS anon_1
If we didn’t call add_entity(User)
, but still asked
contains_eager()
to load the User
entity, it would be
forced to add the table on the outside without the correct
join criteria - note the anon1, "user"
phrase at
the end:
-- incorrect query
SELECT anon_1.address_id AS anon_1_address_id,
anon_1.address_email AS anon_1_address_email,
anon_1.address_user_id AS anon_1_address_user_id,
"user".id AS user_id,
"user".name AS user_name
FROM (
SELECT address.id AS address_id,
address.email AS address_email,
address.user_id AS address_user_id
FROM address JOIN "user" ON "user".id = address.user_id
WHERE "user".name LIKE :name_1) AS anon_1, "user"
*entities¶ – optional list of entities which will replace those being selected.
sqlalchemy.orm.query.Query.
from_statement
(statement)¶Execute the given SELECT statement and return results.
This method bypasses all internal statement compilation, and the statement is executed without modification.
The statement is typically either a text()
or select()
construct, and should return the set
of columns
appropriate to the entity class represented by this
Query
.
See also
Using Textual SQL - usage examples in the ORM tutorial
sqlalchemy.orm.query.Query.
get
(ident)¶Return an instance based on the given primary key identifier,
or None
if not found.
E.g.:
my_user = session.query(User).get(5)
some_object = session.query(VersionedFoo).get((5, 10))
some_object = session.query(VersionedFoo).get(
{"id": 5, "version_id": 10})
Query.get()
is special in that it provides direct
access to the identity map of the owning Session
.
If the given primary key identifier is present
in the local identity map, the object is returned
directly from this collection and no SQL is emitted,
unless the object has been marked fully expired.
If not present,
a SELECT is performed in order to locate the object.
Query.get()
also will perform a check if
the object is present in the identity map and
marked as expired - a SELECT
is emitted to refresh the object as well as to
ensure that the row is still present.
If not, ObjectDeletedError
is raised.
Query.get()
is only used to return a single
mapped instance, not multiple instances or
individual column constructs, and strictly
on a single primary key value. The originating
Query
must be constructed in this way,
i.e. against a single mapped entity,
with no additional filtering criterion. Loading
options via Query.options()
may be applied
however, and will be used if the object is not
yet locally present.
A lazy-loading, many-to-one attribute configured
by relationship()
, using a simple
foreign-key-to-primary-key criterion, will also use an
operation equivalent to Query.get()
in order to retrieve
the target value from the local identity map
before querying the database. See Relationship Loading Techniques
for further details on relationship loading.
ident¶ –
A scalar, tuple, or dictionary representing the primary key. For a composite (e.g. multiple column) primary key, a tuple or dictionary should be passed.
For a single-column primary key, the scalar calling form is typically the most expedient. If the primary key of a row is the value “5”, the call looks like:
my_object = query.get(5)
The tuple form contains primary key values typically in
the order in which they correspond to the mapped
Table
object’s primary key columns, or if the
Mapper.primary_key
configuration parameter were
used, in
the order used for that parameter. For example, if the primary key
of a row is represented by the integer
digits “5, 10” the call would look like:
my_object = query.get((5, 10))
The dictionary form should include as keys the mapped attribute names
corresponding to each element of the primary key. If the mapped class
has the attributes id
, version_id
as the attributes which
store the object’s primary key value, the call would look like:
my_object = query.get({"id": 5, "version_id": 10})
New in version 1.3: the Query.get()
method now optionally
accepts a dictionary of attribute names to values in order to
indicate a primary key identifier.
The object instance, or None
.
sqlalchemy.orm.query.Query.
get_execution_options
()¶Get the non-SQL options which will take effect during execution.
New in version 1.3.
See also
sqlalchemy.orm.query.Query.
group_by
(*criterion)¶Apply one or more GROUP BY criterion to the query and return
the newly resulting Query
.
All existing GROUP BY settings can be suppressed by
passing None
- this will suppress any GROUP BY configured
on mappers as well.
New in version 1.1: GROUP BY can be cancelled by passing
None
, in the same way as ORDER BY.
sqlalchemy.orm.query.Query.
having
(criterion)¶Apply a HAVING criterion to the query and return the
newly resulting Query
.
Query.having()
is used in conjunction with
Query.group_by()
.
HAVING criterion makes it possible to use filters on aggregate functions like COUNT, SUM, AVG, MAX, and MIN, eg.:
q = session.query(User.id).\
join(User.addresses).\
group_by(User.id).\
having(func.count(Address.id) > 2)
sqlalchemy.orm.query.Query.
instances
(cursor, _Query__context=None)¶Given a ResultProxy cursor as returned by connection.execute(), return an ORM result as an iterator.
e.g.:
result = engine.execute("select * from users")
for u in session.query(User).instances(result):
print u
sqlalchemy.orm.query.Query.
intersect
(*q)¶Produce an INTERSECT of this Query against one or more queries.
Works the same way as Query.union()
. See
that method for usage examples.
sqlalchemy.orm.query.Query.
intersect_all
(*q)¶Produce an INTERSECT ALL of this Query against one or more queries.
Works the same way as Query.union()
. See
that method for usage examples.
sqlalchemy.orm.query.Query.
is_single_entity
¶Indicates if this Query
returns tuples or single entities.
Returns True if this query returns a single entity for each instance in its result list, and False if this query returns a tuple of entities for each result.
New in version 1.3.11.
See also
sqlalchemy.orm.query.Query.
join
(*props, **kwargs)¶Create a SQL JOIN against this Query
object’s criterion
and apply generatively, returning the newly resulting
Query
.
Simple Relationship Joins
Consider a mapping between two classes User
and Address
,
with a relationship User.addresses
representing a collection
of Address
objects associated with each User
. The most
common usage of Query.join()
is to create a JOIN along this
relationship, using the User.addresses
attribute as an indicator
for how this should occur:
q = session.query(User).join(User.addresses)
Where above, the call to Query.join()
along
User.addresses
will result in SQL approximately equivalent to:
SELECT user.id, user.name
FROM user JOIN address ON user.id = address.user_id
In the above example we refer to User.addresses
as passed to
Query.join()
as the “on clause”, that is, it indicates
how the “ON” portion of the JOIN should be constructed.
To construct a chain of joins, multiple Query.join()
calls may be used. The relationship-bound attribute implies both
the left and right side of the join at once:
q = session.query(User).\
join(User.orders).\
join(Order.items).\
join(Item.keywords)
Note
as seen in the above example, the order in which each
call to the join() method occurs is important. Query would not,
for example, know how to join correctly if we were to specify
User
, then Item
, then Order
, in our chain of joins; in
such a case, depending on the arguments passed, it may raise an
error that it doesn’t know how to join, or it may produce invalid
SQL in which case the database will raise an error. In correct
practice, the
Query.join()
method is invoked in such a way that lines
up with how we would want the JOIN clauses in SQL to be
rendered, and each call should represent a clear link from what
precedes it.
Joins to a Target Entity or Selectable
A second form of Query.join()
allows any mapped entity or
core selectable construct as a target. In this usage,
Query.join()
will attempt to create a JOIN along the
natural foreign key relationship between two entities:
q = session.query(User).join(Address)
In the above calling form, Query.join()
is called upon to
create the “on clause” automatically for us. This calling form will
ultimately raise an error if either there are no foreign keys between
the two entities, or if there are multiple foreign key linkages between
the target entity and the entity or entities already present on the
left side such that creating a join requires more information. Note
that when indicating a join to a target without any ON clause, ORM
configured relationships are not taken into account.
Joins to a Target with an ON Clause
The third calling form allows both the target entity as well as the ON clause to be passed explicitly. A example that includes a SQL expression as the ON clause is as follows:
q = session.query(User).join(Address, User.id==Address.user_id)
The above form may also use a relationship-bound attribute as the ON clause as well:
q = session.query(User).join(Address, User.addresses)
The above syntax can be useful for the case where we wish
to join to an alias of a particular target entity. If we wanted
to join to Address
twice, it could be achieved using two
aliases set up using the aliased()
function:
a1 = aliased(Address)
a2 = aliased(Address)
q = session.query(User).\
join(a1, User.addresses).\
join(a2, User.addresses).\
filter(a1.email_address=='ed@foo.com').\
filter(a2.email_address=='ed@bar.com')
The relationship-bound calling form can also specify a target entity
using the PropComparator.of_type()
method; a query
equivalent to the one above would be:
a1 = aliased(Address)
a2 = aliased(Address)
q = session.query(User).\
join(User.addresses.of_type(a1)).\
join(User.addresses.of_type(a2)).\
filter(a1.email_address == 'ed@foo.com').\
filter(a2.email_address == 'ed@bar.com')
Joining to Tables and Subqueries
The target of a join may also be any table or SELECT statement,
which may be related to a target entity or not. Use the
appropriate .subquery()
method in order to make a subquery
out of a query:
subq = session.query(Address).\
filter(Address.email_address == 'ed@foo.com').\
subquery()
q = session.query(User).join(
subq, User.id == subq.c.user_id
)
Joining to a subquery in terms of a specific relationship and/or
target entity may be achieved by linking the subquery to the
entity using aliased()
:
subq = session.query(Address).\
filter(Address.email_address == 'ed@foo.com').\
subquery()
address_subq = aliased(Address, subq)
q = session.query(User).join(
User.addresses.of_type(address_subq)
)
Controlling what to Join From
In cases where the left side of the current state of
Query
is not in line with what we want to join from,
the Query.select_from()
method may be used:
q = session.query(Address).select_from(User).\
join(User.addresses).\
filter(User.name == 'ed')
Which will produce SQL similar to:
SELECT address.* FROM user
JOIN address ON user.id=address.user_id
WHERE user.name = :name_1
Legacy Features of Query.join()
The Query.join()
method currently supports several
usage patterns and arguments that are considered to be legacy
as of SQLAlchemy 1.3. A deprecation path will follow
in the 1.4 series for the following features:
Joining on relationship names rather than attributes:
session.query(User).join("addresses")
Why it’s legacy: the string name does not provide enough context
for Query.join()
to always know what is desired,
notably in that there is no indication of what the left side
of the join should be. This gives rise to flags like
from_joinpoint
as well as the ability to place several
join clauses in a single Query.join()
call
which don’t solve the problem fully while also
adding new calling styles that are unnecessary and expensive to
accommodate internally.
Modern calling pattern: Use the actual relationship,
e.g. User.addresses
in the above case:
session.query(User).join(User.addresses)
Automatic aliasing with the aliased=True
flag:
session.query(Node).join(Node.children, aliased=True).\
filter(Node.name == 'some name')
Why it’s legacy: the automatic aliasing feature of
Query
is intensely complicated, both in its internal
implementation as well as in its observed behavior, and is almost
never used. It is difficult to know upon inspection where and when
its aliasing of a target entity, Node
in the above case, will be
applied and when it won’t, and additionally the feature has to use
very elaborate heuristics to achieve this implicit behavior.
Modern calling pattern: Use the aliased()
construct
explicitly:
from sqlalchemy.orm import aliased
n1 = aliased(Node)
session.query(Node).join(Node.children.of_type(n1)).\
filter(n1.name == 'some name')
Multiple joins in one call:
session.query(User).join("orders", "items")
session.query(User).join(User.orders, Order.items)
session.query(User).join(
(Order, User.orders),
(Item, Item.order_id == Order.id)
)
# ... and several more forms actually
Why it’s legacy: being able to chain multiple ON clauses in one
call to Query.join()
is yet another attempt to solve
the problem of being able to specify what entity to join from,
and is the source of a large variety of potential calling patterns
that are internally expensive and complicated to parse and
accommodate.
Modern calling pattern: Use relationship-bound attributes
or SQL-oriented ON clauses within separate calls, so that
each call to Query.join()
knows what the left
side should be:
session.query(User).join(User.orders).join(
Item, Item.order_id == Order.id)
*props¶ – Incoming arguments for Query.join()
,
the props collection in modern use should be considered to be a one
or two argument form, either as a single “target” entity or ORM
attribute-bound relationship, or as a target entity plus an “on
clause” which may be a SQL expression or ORM attribute-bound
relationship.
isouter=False¶ – If True, the join used will be a left outer join,
just as if the Query.outerjoin()
method were called.
full=False¶ –
render FULL OUTER JOIN; implies isouter
.
New in version 1.1.
from_joinpoint=False¶ –
When using aliased=True
, a setting
of True here will cause the join to be from the most recent
joined target, rather than starting back from the original
FROM clauses of the query.
Note
This flag is considered legacy.
aliased=False¶ –
If True, indicate that the JOIN target should be
anonymously aliased. Subsequent calls to Query.filter()
and similar will adapt the incoming criterion to the target
alias, until Query.reset_joinpoint()
is called.
Note
This flag is considered legacy.
See also
Querying with Joins in the ORM tutorial.
Mapping Class Inheritance Hierarchies for details on how
Query.join()
is used for inheritance relationships.
join()
- a standalone ORM-level join function,
used internally by Query.join()
, which in previous
SQLAlchemy versions was the primary ORM-level joining interface.
sqlalchemy.orm.query.Query.
label
(name)¶Return the full SELECT statement represented by this
Query
, converted
to a scalar subquery with a label of the given name.
Analogous to SelectBase.label()
.
sqlalchemy.orm.query.Query.
lazy_loaded_from
= None¶An InstanceState
that is using this Query
for a lazy load operation.
The primary rationale for this attribute is to support the horizontal sharding extension, where it is available within specific query execution time hooks created by this extension. To that end, the attribute is only intended to be meaningful at query execution time, and importantly not any time prior to that, including query compilation time.
Note
Within the realm of regular Query
usage,
this attribute is
set by the lazy loader strategy before the query is invoked. However
there is no established hook that is available to reliably intercept
this value programmatically. It is set by the lazy loading strategy
after any mapper option objects would have been applied, and now that
the lazy loading strategy in the ORM makes use of “baked” queries to
cache SQL compilation, the QueryEvents.before_compile()
hook is
also not reliable.
Currently, setting the relationship.bake_queries
to
False
on the target relationship()
,
and then making use of
the QueryEvents.before_compile()
event hook, is the only
available programmatic path to intercepting this attribute. In future
releases, there will be new hooks available that allow interception of
the Query
before it is executed,
rather than before it is
compiled.
New in version 1.2.9.
sqlalchemy.orm.query.Query.
limit
(limit)¶Apply a LIMIT
to the query and return the newly resulting
Query
.
sqlalchemy.orm.query.Query.
merge_result
(iterator, load=True)¶Merge a result into this Query
object’s Session.
Given an iterator returned by a Query
of the same structure
as this one, return an identical iterator of results, with all mapped
instances merged into the session using Session.merge()
. This
is an optimized method which will merge all mapped instances,
preserving the structure of the result rows and unmapped columns with
less method overhead than that of calling Session.merge()
explicitly for each value.
The structure of the results is determined based on the column list of
this Query
- if these do not correspond,
unchecked errors
will occur.
The ‘load’ argument is the same as that of Session.merge()
.
For an example of how Query.merge_result()
is used, see
the source code for the example Dogpile Caching, where
Query.merge_result()
is used to efficiently restore state
from a cache back into a target Session
.
sqlalchemy.orm.query.Query.
offset
(offset)¶Apply an OFFSET
to the query and return the newly resulting
Query
.
sqlalchemy.orm.query.Query.
one
()¶Return exactly one result or raise an exception.
Raises sqlalchemy.orm.exc.NoResultFound
if the query selects
no rows. Raises sqlalchemy.orm.exc.MultipleResultsFound
if multiple object identities are returned, or if multiple
rows are returned for a query that returns only scalar values
as opposed to full identity-mapped entities.
Calling one()
results in an execution of the underlying query.
sqlalchemy.orm.query.Query.
one_or_none
()¶Return at most one result or raise an exception.
Returns None
if the query selects
no rows. Raises sqlalchemy.orm.exc.MultipleResultsFound
if multiple object identities are returned, or if multiple
rows are returned for a query that returns only scalar values
as opposed to full identity-mapped entities.
Calling Query.one_or_none()
results in an execution of the
underlying query.
New in version 1.0.9: Added Query.one_or_none()
sqlalchemy.orm.query.Query.
only_return_tuples
(value)¶When set to True, the query results will always be a tuple.
This is specifically for single element queries. The default is False.
New in version 1.2.5.
See also
sqlalchemy.orm.query.Query.
options
(*args)¶Return a new Query
object,
applying the given list of
mapper options.
Most supplied options regard changing how column- and relationship-mapped attributes are loaded.
sqlalchemy.orm.query.Query.
order_by
(*criterion)¶Apply one or more ORDER BY criterion to the query and return
the newly resulting Query
.
All existing ORDER BY settings can be suppressed by
passing None
- this will suppress any ordering configured
on the mapper()
object using the deprecated
mapper.order_by
parameter.
sqlalchemy.orm.query.Query.
outerjoin
(*props, **kwargs)¶Create a left outer join against this Query
object’s criterion and apply generatively, returning the newly
resulting Query
.
Usage is the same as the Query.join()
method.
sqlalchemy.orm.query.Query.
params
(*args, **kwargs)¶Add values for bind parameters which may have been specified in filter().
Parameters may be specified using **kwargs, or optionally a single dictionary as the first positional argument. The reason for both is that **kwargs is convenient, however some parameter dictionaries contain unicode keys in which case **kwargs cannot be used.
sqlalchemy.orm.query.Query.
populate_existing
()¶Return a Query
that will expire and refresh all instances
as they are loaded, or reused from the current Session
.
populate_existing()
does not improve behavior when
the ORM is used normally - the Session
object’s usual
behavior of maintaining a transaction and expiring all attributes
after rollback or commit handles object state automatically.
This method is not intended for general use.
See also
Refreshing / Expiring - in the ORM Session
documentation
sqlalchemy.orm.query.Query.
prefix_with
(*prefixes)¶Apply the prefixes to the query and return the newly resulting
Query
.
*prefixes¶ – optional prefixes, typically strings, not using any commas. In particular is useful for MySQL keywords and optimizer hints:
e.g.:
query = sess.query(User.name).\
prefix_with('HIGH_PRIORITY').\
prefix_with('SQL_SMALL_RESULT', 'ALL').\
prefix_with('/*+ BKA(user) */')
Would render:
SELECT HIGH_PRIORITY SQL_SMALL_RESULT ALL /*+ BKA(user) */
users.name AS users_name FROM users
See also
sqlalchemy.orm.query.Query.
reset_joinpoint
()¶Return a new Query
, where the “join point” has
been reset back to the base FROM entities of the query.
This method is usually used in conjunction with the
aliased=True
feature of the Query.join()
method. See the example in Query.join()
for how
this is used.
sqlalchemy.orm.query.Query.
scalar
()¶Return the first element of the first result or None if no rows present. If multiple rows are returned, raises MultipleResultsFound.
>>> session.query(Item).scalar()
<Item>
>>> session.query(Item.id).scalar()
1
>>> session.query(Item.id).filter(Item.id < 0).scalar()
None
>>> session.query(Item.id, Item.name).scalar()
1
>>> session.query(func.count(Parent.id)).scalar()
20
This results in an execution of the underlying query.
sqlalchemy.orm.query.Query.
select_entity_from
(from_obj)¶Set the FROM clause of this Query
to a
core selectable, applying it as a replacement FROM clause
for corresponding mapped entities.
The Query.select_entity_from()
method supplies an alternative
approach to the use case of applying an aliased()
construct
explicitly throughout a query. Instead of referring to the
aliased()
construct explicitly,
Query.select_entity_from()
automatically adapts all
occurrences of the entity to the target selectable.
Given a case for aliased()
such as selecting User
objects from a SELECT statement:
select_stmt = select([User]).where(User.id == 7)
user_alias = aliased(User, select_stmt)
q = session.query(user_alias).\
filter(user_alias.name == 'ed')
Above, we apply the user_alias
object explicitly throughout the
query. When it’s not feasible for user_alias
to be referenced
explicitly in many places, Query.select_entity_from()
may be
used at the start of the query to adapt the existing User
entity:
q = session.query(User).\
select_entity_from(select_stmt).\
filter(User.name == 'ed')
Above, the generated SQL will show that the User
entity is
adapted to our statement, even in the case of the WHERE clause:
SELECT anon_1.id AS anon_1_id, anon_1.name AS anon_1_name
FROM (SELECT "user".id AS id, "user".name AS name
FROM "user"
WHERE "user".id = :id_1) AS anon_1
WHERE anon_1.name = :name_1
The Query.select_entity_from()
method is similar to the
Query.select_from()
method,
in that it sets the FROM clause
of the query. The difference is that it additionally applies
adaptation to the other parts of the query that refer to the
primary entity. If above we had used Query.select_from()
instead, the SQL generated would have been:
-- uses plain select_from(), not select_entity_from()
SELECT "user".id AS user_id, "user".name AS user_name
FROM "user", (SELECT "user".id AS id, "user".name AS name
FROM "user"
WHERE "user".id = :id_1) AS anon_1
WHERE "user".name = :name_1
To supply textual SQL to the Query.select_entity_from()
method,
we can make use of the text()
construct. However,
the
text()
construct needs to be aligned with the columns of our
entity, which is achieved by making use of the
TextClause.columns()
method:
text_stmt = text("select id, name from user").columns(
User.id, User.name)
q = session.query(User).select_entity_from(text_stmt)
Query.select_entity_from()
itself accepts an
aliased()
object, so that the special options of aliased()
such as
aliased.adapt_on_names
may be used within the
scope of the Query.select_entity_from()
method’s adaptation
services. Suppose
a view user_view
also returns rows from user
. If
we reflect this view into a Table
, this view has no
relationship to the Table
to which we are mapped,
however
we can use name matching to select from it:
user_view = Table('user_view', metadata,
autoload_with=engine)
user_view_alias = aliased(
User, user_view, adapt_on_names=True)
q = session.query(User).\
select_entity_from(user_view_alias).\
order_by(User.name)
Changed in version 1.1.7: The Query.select_entity_from()
method now accepts an aliased()
object as an alternative
to a FromClause
object.
from_obj¶ – a FromClause
object that will replace
the FROM clause of this Query
.
It also may be an instance
of aliased()
.
See also
sqlalchemy.orm.query.Query.
select_from
(*from_obj)¶Set the FROM clause of this Query
explicitly.
Query.select_from()
is often used in conjunction with
Query.join()
in order to control which entity is selected
from on the “left” side of the join.
The entity or selectable object here effectively replaces the
“left edge” of any calls to Query.join()
, when no
joinpoint is otherwise established - usually, the default “join
point” is the leftmost entity in the Query
object’s
list of entities to be selected.
A typical example:
q = session.query(Address).select_from(User).\
join(User.addresses).\
filter(User.name == 'ed')
Which produces SQL equivalent to:
SELECT address.* FROM user
JOIN address ON user.id=address.user_id
WHERE user.name = :name_1
*from_obj¶ – collection of one or more entities to apply
to the FROM clause. Entities can be mapped classes,
AliasedClass
objects, Mapper
objects
as well as core FromClause
elements like subqueries.
Changed in version 0.9: This method no longer applies the given FROM object
to be the selectable from which matching entities
select from; the select_entity_from()
method
now accomplishes this. See that method for a description
of this behavior.
sqlalchemy.orm.query.Query.
selectable
¶Return the Select
object emitted by this
Query
.
Used for inspect()
compatibility, this is equivalent to:
query.enable_eagerloads(False).with_labels().statement
sqlalchemy.orm.query.Query.
slice
(start, stop)¶Computes the “slice” of the Query
represented by
the given indices and returns the resulting Query
.
The start and stop indices behave like the argument to Python’s
built-in range()
function. This method provides an
alternative to using LIMIT
/OFFSET
to get a slice of the
query.
For example,
session.query(User).order_by(User.id).slice(1, 3)
renders as
SELECT users.id AS users_id,
users.name AS users_name
FROM users ORDER BY users.id
LIMIT ? OFFSET ?
(2, 1)
sqlalchemy.orm.query.Query.
statement
¶The full SELECT statement represented by this Query.
The statement by default will not have disambiguating labels applied to the construct unless with_labels(True) is called first.
sqlalchemy.orm.query.Query.
subquery
(name=None, with_labels=False, reduce_columns=False)¶Return the full SELECT statement represented by
this Query
, embedded within an
Alias
.
Eager JOIN generation within the query is disabled.
name¶ – string name to be assigned as the alias;
this is passed through to FromClause.alias()
.
If None
, a name will be deterministically generated
at compile time.
with_labels¶ – if True, with_labels()
will be called
on the Query
first to apply table-qualified labels
to all columns.
reduce_columns¶ – if True,
Select.reduce_columns()
will
be called on the resulting select()
construct,
to remove same-named columns where one also refers to the other
via foreign key or WHERE clause equivalence.
sqlalchemy.orm.query.Query.
suffix_with
(*suffixes)¶Apply the suffix to the query and return the newly resulting
Query
.
*suffixes¶ – optional suffixes, typically strings, not using any commas.
New in version 1.0.0.
sqlalchemy.orm.query.Query.
union
(*q)¶Produce a UNION of this Query against one or more queries.
e.g.:
q1 = sess.query(SomeClass).filter(SomeClass.foo=='bar')
q2 = sess.query(SomeClass).filter(SomeClass.bar=='foo')
q3 = q1.union(q2)
The method accepts multiple Query objects so as to control
the level of nesting. A series of union()
calls such as:
x.union(y).union(z).all()
will nest on each union()
, and produces:
SELECT * FROM (SELECT * FROM (SELECT * FROM X UNION
SELECT * FROM y) UNION SELECT * FROM Z)
Whereas:
x.union(y, z).all()
produces:
SELECT * FROM (SELECT * FROM X UNION SELECT * FROM y UNION
SELECT * FROM Z)
Note that many database backends do not allow ORDER BY to
be rendered on a query called within UNION, EXCEPT, etc.
To disable all ORDER BY clauses including those configured
on mappers, issue query.order_by(None)
- the resulting
Query
object will not render ORDER BY within
its SELECT statement.
sqlalchemy.orm.query.Query.
union_all
(*q)¶Produce a UNION ALL of this Query against one or more queries.
Works the same way as Query.union()
. See
that method for usage examples.
sqlalchemy.orm.query.Query.
update
(values, synchronize_session='evaluate', update_args=None)¶Perform a bulk update query.
Updates rows matched by this query in the database.
E.g.:
sess.query(User).filter(User.age == 25).\
update({User.age: User.age - 10}, synchronize_session=False)
sess.query(User).filter(User.age == 25).\
update({"age": User.age - 10}, synchronize_session='evaluate')
Warning
The Query.update()
method is a “bulk” operation,
which bypasses ORM unit-of-work automation in favor of greater
performance. Please read all caveats and warnings below.
values¶ –
a dictionary with attributes names, or alternatively
mapped attributes or SQL expressions, as keys, and literal
values or sql expressions as values. If parameter-ordered
mode is desired, the values can be
passed as a list of 2-tuples;
this requires that the
update.preserve_parameter_order
flag is passed to the Query.update.update_args
dictionary
as well.
Changed in version 1.0.0: - string names in the values dictionary are now resolved against the mapped entity; previously, these strings were passed as literal column names with no mapper-level translation.
synchronize_session¶ –
chooses the strategy to update the attributes on objects in the session. Valid values are:
False
- don’t synchronize the session. This option is the most efficient and is reliable once the session is expired, which typically occurs after a commit(), or explicitly using expire_all(). Before the expiration, updated objects may still remain in the session with stale values on their attributes, which can lead to confusing results.
'fetch'
- performs a select query before the update to find objects that are matched by the update query. The updated attributes are expired on matched objects.
'evaluate'
- Evaluate the Query’s criteria in Python straight on the objects in the session. If evaluation of the criteria isn’t implemented, an exception is raised.The expression evaluator currently doesn’t account for differing string collations between the database and Python.
update_args¶ –
Optional dictionary, if present will be passed
to the underlying update()
construct as the **kw
for
the object. May be used to pass dialect-specific arguments such
as mysql_limit
, as well as other special arguments such as
update.preserve_parameter_order
.
New in version 1.0.0.
the count of rows matched as returned by the database’s “row count” feature.
Warning
Additional Caveats for bulk query updates
The method does not offer in-Python cascading of relationships - it is assumed that ON UPDATE CASCADE is configured for any foreign key references which require it, otherwise the database may emit an integrity violation if foreign key references are being enforced.
After the UPDATE, dependent objects in the
Session
which were impacted by an ON UPDATE
CASCADE may not contain the current state; this issue is
resolved once the Session
is expired, which
normally occurs upon Session.commit()
or can be
forced by using Session.expire_all()
.
The 'fetch'
strategy results in an additional
SELECT statement emitted and will significantly reduce
performance.
The 'evaluate'
strategy performs a scan of
all matching objects within the Session
; if the
contents of the Session
are expired, such as
via a proceeding Session.commit()
call, this will
result in SELECT queries emitted for every matching object.
The method supports multiple table updates, as detailed
in Multiple Table Updates, and this behavior does
extend to support updates of joined-inheritance and
other multiple table mappings. However, the join
condition of an inheritance mapper is not
automatically rendered. Care must be taken in any
multiple-table update to explicitly include the joining
condition between those tables, even in mappings where
this is normally automatic. E.g. if a class Engineer
subclasses Employee
, an UPDATE of the Engineer
local table using criteria against the Employee
local table might look like:
session.query(Engineer).\
filter(Engineer.id == Employee.id).\
filter(Employee.name == 'dilbert').\
update({"engineer_type": "programmer"})
The polymorphic identity WHERE criteria is not included for single- or joined- table updates - this must be added manually, even for single table inheritance.
The MapperEvents.before_update()
and
MapperEvents.after_update()
events are not invoked from this method. Instead, the
SessionEvents.after_bulk_update()
method is provided to
act upon a mass UPDATE of entity rows.
sqlalchemy.orm.query.Query.
value
(column)¶Return a scalar result corresponding to the given column expression.
sqlalchemy.orm.query.Query.
values
(*columns)¶Return an iterator yielding result tuples corresponding to the given list of columns.
sqlalchemy.orm.query.Query.
whereclause
¶A readonly attribute which returns the current WHERE criterion for this Query.
This returned value is a SQL expression construct, or None
if no
criterion has been established.
sqlalchemy.orm.query.Query.
with_entities
(*entities)¶Return a new Query
replacing the SELECT list with the
given entities.
e.g.:
# Users, filtered on some arbitrary criterion
# and then ordered by related email address
q = session.query(User).\
join(User.address).\
filter(User.name.like('%ed%')).\
order_by(Address.email)
# given *only* User.id==5, Address.email, and 'q', what
# would the *next* User in the result be ?
subq = q.with_entities(Address.email).\
order_by(None).\
filter(User.id==5).\
subquery()
q = q.join((subq, subq.c.email < Address.email)).\
limit(1)
sqlalchemy.orm.query.Query.
with_for_update
(read=False, nowait=False, of=None, skip_locked=False, key_share=False)¶return a new Query
with the specified options for the
FOR UPDATE
clause.
The behavior of this method is identical to that of
GenerativeSelect.with_for_update()
.
When called with no arguments,
the resulting SELECT
statement will have a FOR UPDATE
clause
appended. When additional arguments are specified, backend-specific
options such as FOR UPDATE NOWAIT
or LOCK IN SHARE MODE
can take effect.
E.g.:
q = sess.query(User).populate_existing().with_for_update(nowait=True, of=User)
The above query on a PostgreSQL backend will render like:
SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT
New in version 0.9.0: Query.with_for_update()
supersedes
the Query.with_lockmode()
method.
Note
It is generally a good idea to combine the use of the
Query.populate_existing()
method when using the
Query.with_for_update()
method. The purpose of
Query.populate_existing()
is to force all the data read
from the SELECT to be populated into the ORM objects returned,
even if these objects are already in the identity map.
See also
GenerativeSelect.with_for_update()
- Core level method with
full argument and behavioral description.
Query.populate_existing()
- overwrites attributes of
objects already loaded in the identity map.
sqlalchemy.orm.query.Query.
with_hint
(selectable, text, dialect_name='*')¶Add an indexing or other executional context
hint for the given entity or selectable to
this Query
.
Functionality is passed straight through to
Select.with_hint()
,
with the addition that selectable
can be a
Table
, Alias
,
or ORM entity / mapped class
/etc.
See also
Query.prefix_with()
- generic SELECT prefixing which also
can suit some database-specific HINT syntaxes such as MySQL
optimizer hints
sqlalchemy.orm.query.Query.
with_labels
()¶Apply column labels to the return value of Query.statement.
Indicates that this Query’s statement accessor should return a SELECT statement that applies labels to all columns in the form <tablename>_<columnname>; this is commonly used to disambiguate columns from multiple tables which have the same name.
When the Query actually issues SQL to load rows, it always uses column labeling.
Note
The Query.with_labels()
method only applies
the output of Query.statement
, and not to any of
the result-row invoking systems of Query
itself,
e.g. Query.first()
, Query.all()
, etc.
To execute
a query using Query.with_labels()
, invoke the
Query.statement
using Session.execute()
:
result = session.execute(query.with_labels().statement)
sqlalchemy.orm.query.Query.
with_lockmode
(mode)¶Return a new Query
object with the specified “locking mode”,
which essentially refers to the FOR UPDATE
clause.
Deprecated since version 0.9: The Query.with_lockmode()
method is deprecated and will be removed in a future release. Please refer to Query.with_for_update()
.
mode¶ –
a string representing the desired locking mode. Valid values are:
None
- translates to no lockmode
'update'
- translates to FOR UPDATE
(standard SQL, supported by most dialects)
'update_nowait'
- translates to FOR UPDATE NOWAIT
(supported by Oracle, PostgreSQL 8.1 upwards)
'read'
- translates to LOCK IN SHARE MODE
(for MySQL),
and FOR SHARE
(for PostgreSQL)
See also
Query.with_for_update()
- improved API for
specifying the FOR UPDATE
clause.
sqlalchemy.orm.query.Query.
with_parent
(instance, property=None, from_entity=None)¶Add filtering criterion that relates the given instance
to a child object or collection, using its attribute state
as well as an established relationship()
configuration.
The method uses the with_parent()
function to generate
the clause, the result of which is passed to
Query.filter()
.
Parameters are the same as with_parent()
, with the exception
that the given property can be None, in which case a search is
performed against this Query
object’s target mapper.
instance¶ – An instance which has some relationship()
.
property¶ – String property name, or class-bound attribute, which indicates what relationship from the instance should be used to reconcile the parent/child relationship.
from_entity¶ – Entity in which to consider as the left side. This defaults to the
“zero” entity of the Query
itself.
sqlalchemy.orm.query.Query.
with_polymorphic
(cls_or_mappers, selectable=None, polymorphic_on=None)¶Load columns for inheriting classes.
Query.with_polymorphic()
applies transformations
to the “main” mapped class represented by this Query
.
The “main” mapped class here means the Query
object’s first argument is a full class, i.e.
session.query(SomeClass)
. These transformations allow additional
tables to be present in the FROM clause so that columns for a
joined-inheritance subclass are available in the query, both for the
purposes of load-time efficiency as well as the ability to use
these columns at query time.
See the documentation section Using with_polymorphic for details on how this method is used.
sqlalchemy.orm.query.Query.
with_session
(session)¶Return a Query
that will use the given
Session
.
While the Query
object is normally instantiated using the
Session.query()
method, it is legal to build the
Query
directly without necessarily using a Session
. Such a
Query
object, or any Query
already associated
with a different Session
, can produce a new
Query
object associated with a target session using this method:
from sqlalchemy.orm import Query
query = Query([MyClass]).filter(MyClass.id == 5)
result = query.with_session(my_session).one()
sqlalchemy.orm.query.Query.
with_statement_hint
(text, dialect_name='*')¶Add a statement hint to this Select
.
This method is similar to Select.with_hint()
except that
it does not require an individual table, and instead applies to the
statement as a whole.
This feature calls down into
Select.with_statement_hint()
.
New in version 1.0.0.
See also
sqlalchemy.orm.query.Query.
with_transformation
(fn)¶Return a new Query
object transformed by
the given function.
E.g.:
def filter_something(criterion):
def transform(q):
return q.filter(criterion)
return transform
q = q.with_transformation(filter_something(x==5))
This allows ad-hoc recipes to be created for Query
objects. See the example at Building Transformers.
sqlalchemy.orm.query.Query.
yield_per
(count)¶Yield only count
rows at a time.
The purpose of this method is when fetching very large result sets (> 10K rows), to batch results in sub-collections and yield them out partially, so that the Python interpreter doesn’t need to declare very large areas of memory which is both time consuming and leads to excessive memory use. The performance from fetching hundreds of thousands of rows can often double when a suitable yield-per setting (e.g. approximately 1000) is used, even with DBAPIs that buffer rows (which are most).
The Query.yield_per()
method is not compatible
subqueryload eager loading or joinedload eager loading when
using collections. It is potentially compatible with “select in”
eager loading, provided the database driver supports multiple,
independent cursors (pysqlite and psycopg2 are known to work,
MySQL and SQL Server ODBC drivers do not).
Therefore in some cases, it may be helpful to disable
eager loads, either unconditionally with
Query.enable_eagerloads()
:
q = sess.query(Object).yield_per(100).enable_eagerloads(False)
Or more selectively using lazyload()
; such as with
an asterisk to specify the default loader scheme:
q = sess.query(Object).yield_per(100).\
options(lazyload('*'), joinedload(Object.some_related))
Warning
Use this method with caution; if the same instance is present in more than one batch of rows, end-user changes to attributes will be overwritten.
In particular, it’s usually impossible to use this setting
with eagerly loaded collections (i.e. any lazy=’joined’ or
‘subquery’) since those collections will be cleared for a
new load when encountered in a subsequent result batch.
In the case of ‘subquery’ loading, the full result for all
rows is fetched which generally defeats the purpose of
Query.yield_per()
.
Also note that while
Query.yield_per()
will set the
stream_results
execution option to True, currently
this is only understood by
psycopg2
,
mysqldb
and
pymysql
dialects
which will stream results using server side cursors
instead of pre-buffer all rows for this query. Other
DBAPIs pre-buffer all rows before making them
available. The memory use of raw database rows is much less
than that of an ORM-mapped object, but should still be taken into
consideration when benchmarking.
See also
Object Name | Description |
---|---|
|
Produce an alias of the given element, usually an |
Represents an “aliased” form of a mapped class for usage with Query. |
|
Provide an inspection interface for an
|
|
A grouping of SQL expressions that are returned by a
|
|
|
Produce an inner join between left and right clauses. |
|
|
Represents loader options which modify the state of a
|
|
|
Produce a left outer join between left and right clauses. |
|
Create filtering criterion that relates this query’s primary entity
to the given related instance, using established
|
sqlalchemy.orm.
aliased
(element, alias=None, name=None, flat=False, adapt_on_names=False)¶Produce an alias of the given element, usually an AliasedClass
instance.
E.g.:
my_alias = aliased(MyClass)
session.query(MyClass, my_alias).filter(MyClass.id > my_alias.id)
The aliased()
function is used to create an ad-hoc mapping of a
mapped class to a new selectable. By default, a selectable is generated
from the normally mapped selectable (typically a Table
)
using the
FromClause.alias()
method. However, aliased()
can also be
used to link the class to a new select()
statement.
Also, the with_polymorphic()
function is a variant of
aliased()
that is intended to specify a so-called “polymorphic
selectable”, that corresponds to the union of several joined-inheritance
subclasses at once.
For convenience, the aliased()
function also accepts plain
FromClause
constructs, such as a
Table
or
select()
construct. In those cases, the
FromClause.alias()
method is called on the object and the new
Alias
object returned. The returned
Alias
is not
ORM-mapped in this case.
element¶ – element to be aliased. Is normally a mapped class,
but for convenience can also be a FromClause
element.
alias¶ – Optional selectable unit to map the element to. This is
usually used to link the object to a subquery, and should be an aliased
select construct as one would produce from the
Query.subquery()
method or
the Select.alias()
methods of the
select()
construct.
name¶ – optional string name to use for the alias, if not specified
by the alias
parameter. The name, among other things, forms the
attribute name that will be accessible via tuples returned by a
Query
object.
flat¶ –
Boolean, will be passed through to the
FromClause.alias()
call so that aliases of
Join
objects
don’t include an enclosing SELECT. This can lead to more efficient
queries in many circumstances. A JOIN against a nested JOIN will be
rewritten as a JOIN against an aliased SELECT subquery on backends that
don’t support this syntax.
See also
adapt_on_names¶ –
if True, more liberal “matching” will be used when mapping the mapped columns of the ORM entity to those of the given selectable - a name-based match will be performed if the given selectable doesn’t otherwise have a column that corresponds to one on the entity. The use case for this is when associating an entity with some derived selectable such as one that uses aggregate functions:
class UnitPrice(Base):
__tablename__ = 'unit_price'
...
unit_id = Column(Integer)
price = Column(Numeric)
aggregated_unit_price = Session.query(
func.sum(UnitPrice.price).label('price')
).group_by(UnitPrice.unit_id).subquery()
aggregated_unit_price = aliased(UnitPrice,
alias=aggregated_unit_price, adapt_on_names=True)
Above, functions on aggregated_unit_price
which refer to
.price
will return the
func.sum(UnitPrice.price).label('price')
column, as it is
matched on the name “price”. Ordinarily, the “price” function
wouldn’t have any “column correspondence” to the actual
UnitPrice.price
column as it is not a proxy of the original.
sqlalchemy.orm.util.
AliasedClass
(cls, alias=None, name=None, flat=False, adapt_on_names=False, with_polymorphic_mappers=(), with_polymorphic_discriminator=None, base_alias=None, use_mapper_path=False, represents_outer_join=False)¶Represents an “aliased” form of a mapped class for usage with Query.
The ORM equivalent of a alias()
construct, this object mimics the mapped class using a
__getattr__
scheme and maintains a reference to a
real Alias
object.
A primary purpose of AliasedClass
is to serve as an alternate
within a SQL statement generated by the ORM, such that an existing
mapped entity can be used in multiple contexts. A simple example:
# find all pairs of users with the same name
user_alias = aliased(User)
session.query(User, user_alias).\
join((user_alias, User.id > user_alias.id)).\
filter(User.name == user_alias.name)
AliasedClass
is also capable of mapping an existing mapped
class to an entirely new selectable, provided this selectable is column-
compatible with the existing mapped selectable, and it can also be
configured in a mapping as the target of a relationship()
.
See the links below for examples.
The AliasedClass
object is constructed typically using the
aliased()
function. It also is produced with additional
configuration when using the with_polymorphic()
function.
The resulting object is an instance of AliasedClass
.
This object implements an attribute scheme which produces the
same attribute and method interface as the original mapped
class, allowing AliasedClass
to be compatible
with any attribute technique which works on the original class,
including hybrid attributes (see Hybrid Attributes).
The AliasedClass
can be inspected for its underlying
Mapper
, aliased selectable, and other information
using inspect()
:
from sqlalchemy import inspect
my_alias = aliased(MyClass)
insp = inspect(my_alias)
The resulting inspection object is an instance of AliasedInsp
.
sqlalchemy.orm.util.
AliasedInsp
(entity, mapper, selectable, name, with_polymorphic_mappers, polymorphic_on, _base_alias, _use_mapper_path, adapt_on_names, represents_outer_join)¶Provide an inspection interface for an
AliasedClass
object.
The AliasedInsp
object is returned
given an AliasedClass
using the
inspect()
function:
from sqlalchemy import inspect
from sqlalchemy.orm import aliased
my_alias = aliased(MyMappedClass)
insp = inspect(my_alias)
Attributes on AliasedInsp
include:
entity
- the AliasedClass
represented.
mapper
- the Mapper
mapping the underlying class.
selectable
- the Alias
construct which ultimately
represents an aliased Table
or
Select
construct.
name
- the name of the alias. Also is used as the attribute
name when returned in a result tuple from Query
.
with_polymorphic_mappers
- collection of Mapper
objects
indicating all those mappers expressed in the select construct
for the AliasedClass
.
polymorphic_on
- an alternate column or SQL expression which
will be used as the “discriminator” for a polymorphic load.
See also
Class signature
class sqlalchemy.orm.util.AliasedInsp
(sqlalchemy.orm.base.InspectionAttr
)
sqlalchemy.orm.query.
Bundle
(name, *exprs, **kw)¶A grouping of SQL expressions that are returned by a
Query
under one namespace.
The Bundle
essentially allows nesting of the tuple-based
results returned by a column-oriented Query
object.
It also
is extensible via simple subclassing, where the primary capability
to override is that of how the set of expressions should be returned,
allowing post-processing as well as custom return types, without
involving ORM identity-mapped classes.
New in version 0.9.0.
See also
Class signature
class sqlalchemy.orm.query.Bundle
(sqlalchemy.orm.base.InspectionAttr
)
sqlalchemy.orm.query.Bundle.
__init__
(name, *exprs, **kw)¶Construct a new Bundle
.
e.g.:
bn = Bundle("mybundle", MyClass.x, MyClass.y)
for row in session.query(bn).filter(
bn.c.x == 5).filter(bn.c.y == 4):
print(row.mybundle.x, row.mybundle.y)
sqlalchemy.orm.query.Bundle.
c
= None¶An alias for Bundle.columns
.
sqlalchemy.orm.query.Bundle.
columns
= None¶A namespace of SQL expressions referred to by this Bundle
.
e.g.:
bn = Bundle("mybundle", MyClass.x, MyClass.y) q = sess.query(bn).filter(bn.c.x == 5)Nesting of bundles is also supported:
b1 = Bundle("b1", Bundle('b2', MyClass.a, MyClass.b), Bundle('b3', MyClass.x, MyClass.y) ) q = sess.query(b1).filter( b1.c.b2.c.a == 5).filter(b1.c.b3.c.y == 9)
See also
sqlalchemy.orm.query.Bundle.
create_row_processor
(query, procs, labels)¶Produce the “row processing” function for this Bundle
.
May be overridden by subclasses.
See also
Column Bundles - includes an example of subclassing.
sqlalchemy.orm.query.Bundle.
label
(name)¶Provide a copy of this Bundle
passing a new label.
sqlalchemy.orm.query.Bundle.
single_entity
= False¶If True, queries for a single Bundle will be returned as a single entity, rather than an element within a keyed tuple.
sqlalchemy.util.
KeyedTuple
(vals, labels=None)¶tuple
subclass that adds labeled names.
E.g.:
>>> k = KeyedTuple([1, 2, 3], labels=["one", "two", "three"])
>>> k.one
1
>>> k.two
2
Result rows returned by Query
that contain multiple
ORM entities and/or column expressions make use of this
class to return rows.
The KeyedTuple
exhibits similar behavior to the
collections.namedtuple()
construct provided in the Python
standard library, however is architected very differently.
Unlike collections.namedtuple()
, KeyedTuple
is
does not rely on creation of custom subtypes in order to represent
a new series of keys, instead each KeyedTuple
instance
receives its list of keys in place. The subtype approach
of collections.namedtuple()
introduces significant complexity
and performance overhead, which is not necessary for the
Query
object’s use case.
See also
Class signature
class sqlalchemy.util.KeyedTuple
(sqlalchemy.util.AbstractKeyedTuple
)
sqlalchemy.util.KeyedTuple.
_asdict
()¶Return the contents of this KeyedTuple
as a dictionary.
This method provides compatibility with collections.namedtuple()
,
with the exception that the dictionary returned is not ordered.
sqlalchemy.util.KeyedTuple.
_fields
¶Return a tuple of string key names for this KeyedTuple
.
This method provides compatibility with collections.namedtuple()
.
See also
sqlalchemy.util.KeyedTuple.
keys
()¶inherited from the AbstractKeyedTuple.keys()
method of AbstractKeyedTuple
Return a list of string key names for this KeyedTuple
.
See also
sqlalchemy.orm.strategy_options.
Load
(entity)¶Represents loader options which modify the state of a
Query
in order to affect how various mapped attributes are
loaded.
The Load
object is in most cases used implicitly behind the
scenes when one makes use of a query option like joinedload()
,
defer()
, or similar. However, the Load
object
can also be used directly, and in some cases can be useful.
To use Load
directly, instantiate it with the target mapped
class as the argument. This style of usage is
useful when dealing with a Query
that has multiple entities:
myopt = Load(MyClass).joinedload("widgets")
The above myopt
can now be used with Query.options()
,
where it
will only take effect for the MyClass
entity:
session.query(MyClass, MyOtherClass).options(myopt)
One case where Load
is useful as public API is when specifying
“wildcard” options that only take effect for a certain class:
session.query(Order).options(Load(Order).lazyload('*'))
Above, all relationships on Order
will be lazy-loaded, but other
attributes on those descendant objects will load using their normal
loader strategy.
Class signature
class sqlalchemy.orm.strategy_options.Load
(sqlalchemy.sql.expression.Generative
, sqlalchemy.orm.MapperOption
)
sqlalchemy.orm.strategy_options.Load.
baked_lazyload
(attr)¶Produce a new Load
object with the
baked_lazyload()
option applied.
See baked_lazyload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
contains_eager
(attr, alias=None)¶Produce a new Load
object with the
contains_eager()
option applied.
See contains_eager()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
defaultload
(attr)¶Produce a new Load
object with the
defaultload()
option applied.
See defaultload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
defer
(key)¶Produce a new Load
object with the
defer()
option applied.
See defer()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
immediateload
(attr)¶Produce a new Load
object with the
immediateload()
option applied.
See immediateload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
joinedload
(attr, innerjoin=None)¶Produce a new Load
object with the
joinedload()
option applied.
See joinedload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
lazyload
(attr)¶Produce a new Load
object with the
lazyload()
option applied.
See lazyload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
load_only
(*attrs)¶Produce a new Load
object with the
load_only()
option applied.
See load_only()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
noload
(attr)¶Produce a new Load
object with the
noload()
option applied.
See noload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
options
(*opts)¶Apply a series of options as sub-options to this
Load
object.
E.g.:
query = session.query(Author)
query = query.options(
joinedload(Author.book).options(
load_only("summary", "excerpt"),
joinedload(Book.citations).options(
joinedload(Citation.author)
)
)
)
*opts¶ – A series of loader option objects (ultimately
Load
objects) which should be applied to the path
specified by this Load
object.
New in version 1.3.6.
sqlalchemy.orm.strategy_options.Load.
process_query
(query)¶Apply a modification to the given Query
.
sqlalchemy.orm.strategy_options.Load.
process_query_conditionally
(query)¶same as process_query(), except that this option may not apply to the given query.
This is typically used during a lazy load or scalar refresh operation to propagate options stated in the original Query to the new Query being used for the load. It occurs for those options that specify propagate_to_loaders=True.
sqlalchemy.orm.strategy_options.Load.
raiseload
(attr, sql_only=False)¶Produce a new Load
object with the
raiseload()
option applied.
See raiseload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
selectin_polymorphic
(classes)¶Produce a new Load
object with the
selectin_polymorphic()
option applied.
See selectin_polymorphic()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
selectinload
(attr)¶Produce a new Load
object with the
selectinload()
option applied.
See selectinload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
subqueryload
(attr)¶Produce a new Load
object with the
subqueryload()
option applied.
See subqueryload()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
undefer
(key)¶Produce a new Load
object with the
undefer()
option applied.
See undefer()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
undefer_group
(name)¶Produce a new Load
object with the
undefer_group()
option applied.
See undefer_group()
for usage examples.
sqlalchemy.orm.strategy_options.Load.
with_expression
(key, expression)¶Produce a new Load
object with the
with_expression()
option applied.
See with_expression()
for usage examples.
sqlalchemy.orm.
join
(left, right, onclause=None, isouter=False, full=False, join_to_left=None)¶Produce an inner join between left and right clauses.
join()
is an extension to the core join interface
provided by join()
, where the
left and right selectables may be not only core selectable
objects such as Table
, but also mapped classes or
AliasedClass
instances. The “on” clause can
be a SQL expression, or an attribute or string name
referencing a configured relationship()
.
join()
is not commonly needed in modern usage,
as its functionality is encapsulated within that of the
Query.join()
method, which features a
significant amount of automation beyond join()
by itself. Explicit usage of join()
with Query
involves usage of the
Query.select_from()
method, as in:
from sqlalchemy.orm import join
session.query(User).\
select_from(join(User, Address, User.addresses)).\
filter(Address.email_address=='foo@bar.com')
In modern SQLAlchemy the above join can be written more succinctly as:
session.query(User).\
join(User.addresses).\
filter(Address.email_address=='foo@bar.com')
See Query.join()
for information on modern usage
of ORM level joins.
Deprecated since version 0.8: the join_to_left
parameter is deprecated, and will be removed
in a future release. The parameter has no effect.
sqlalchemy.orm.
outerjoin
(left, right, onclause=None, full=False, join_to_left=None)¶Produce a left outer join between left and right clauses.
This is the “outer join” version of the join()
function,
featuring the same behavior except that an OUTER JOIN is generated.
See that function’s documentation for other usage details.
sqlalchemy.orm.
with_parent
(instance, prop, from_entity=None)¶Create filtering criterion that relates this query’s primary entity
to the given related instance, using established
relationship()
configuration.
The SQL rendered is the same as that rendered when a lazy loader would fire off from the given parent on that attribute, meaning that the appropriate state is taken from the parent object in Python without the need to render joins to the parent table in the rendered statement.
instance¶ – An instance which has some relationship()
.
property¶ – String property name, or class-bound attribute, which indicates what relationship from the instance should be used to reconcile the parent/child relationship.
from_entity¶ –
Entity in which to consider as the left side. This defaults to the
“zero” entity of the Query
itself.
New in version 1.2.
flambé! the dragon and The Alchemist image designs created and generously donated by Rotem Yaari.
Created using Sphinx 3.5.3.