Module « sqlalchemy.orm »
Signature de la fonction relationship
def relationship(argument, secondary=None, primaryjoin=None, secondaryjoin=None, foreign_keys=None, uselist=None, order_by=False, backref=None, back_populates=None, overlaps=None, post_update=False, cascade=False, viewonly=False, lazy='select', collection_class=None, passive_deletes=False, passive_updates=True, remote_side=None, enable_typechecks=True, join_depth=None, comparator_factory=None, single_parent=False, innerjoin=False, distinct_target_key=None, doc=None, active_history=False, cascade_backrefs=True, load_on_pending=False, bake_queries=True, _local_remote_pairs=None, query_class=None, info=None, omit_join=None, sync_backref=None, _legacy_inactive_history_style=False)
Description
relationship.__doc__
Provide a relationship between two mapped classes.
This corresponds to a parent-child or associative table relationship.
The constructed class is an instance of
:class:`.RelationshipProperty`.
A typical :func:`_orm.relationship`, used in a classical mapping::
mapper(Parent, properties={
'children': relationship(Child)
})
Some arguments accepted by :func:`_orm.relationship`
optionally accept a
callable function, which when called produces the desired value.
The callable is invoked by the parent :class:`_orm.Mapper` at "mapper
initialization" time, which happens only when mappers are first used,
and is assumed to be after all mappings have been constructed. This
can be used to resolve order-of-declaration and other dependency
issues, such as if ``Child`` is declared below ``Parent`` in the same
file::
mapper(Parent, properties={
"children":relationship(lambda: Child,
order_by=lambda: Child.id)
})
When using the :ref:`declarative_toplevel` extension, the Declarative
initializer allows string arguments to be passed to
:func:`_orm.relationship`. These string arguments are converted into
callables that evaluate the string as Python code, using the
Declarative class-registry as a namespace. This allows the lookup of
related classes to be automatic via their string name, and removes the
need for related classes to be imported into the local module space
before the dependent classes have been declared. It is still required
that the modules in which these related classes appear are imported
anywhere in the application at some point before the related mappings
are actually used, else a lookup error will be raised when the
:func:`_orm.relationship`
attempts to resolve the string reference to the
related class. An example of a string- resolved class is as
follows::
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child", order_by="Child.id")
.. seealso::
:ref:`relationship_config_toplevel` - Full introductory and
reference documentation for :func:`_orm.relationship`.
:ref:`orm_tutorial_relationship` - ORM tutorial introduction.
:param argument:
A mapped class, or actual :class:`_orm.Mapper` instance,
representing
the target of the relationship.
:paramref:`_orm.relationship.argument`
may also be passed as a callable
function which is evaluated at mapper initialization time, and may
be passed as a string name when using Declarative.
.. warning:: Prior to SQLAlchemy 1.3.16, this value is interpreted
using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
.. versionchanged 1.3.16::
The string evaluation of the main "argument" no longer accepts an
open ended Python expression, instead only accepting a string
class name or dotted package-qualified name.
.. seealso::
:ref:`declarative_configuring_relationships` - further detail
on relationship configuration when using Declarative.
:param secondary:
For a many-to-many relationship, specifies the intermediary
table, and is typically an instance of :class:`_schema.Table`.
In less common circumstances, the argument may also be specified
as an :class:`_expression.Alias` construct, or even a
:class:`_expression.Join` construct.
:paramref:`_orm.relationship.secondary` may
also be passed as a callable function which is evaluated at
mapper initialization time. When using Declarative, it may also
be a string argument noting the name of a :class:`_schema.Table`
that is
present in the :class:`_schema.MetaData`
collection associated with the
parent-mapped :class:`_schema.Table`.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
The :paramref:`_orm.relationship.secondary` keyword argument is
typically applied in the case where the intermediary
:class:`_schema.Table`
is not otherwise expressed in any direct class mapping. If the
"secondary" table is also explicitly mapped elsewhere (e.g. as in
:ref:`association_pattern`), one should consider applying the
:paramref:`_orm.relationship.viewonly` flag so that this
:func:`_orm.relationship`
is not used for persistence operations which
may conflict with those of the association object pattern.
.. seealso::
:ref:`relationships_many_to_many` - Reference example of "many
to many".
:ref:`orm_tutorial_many_to_many` - ORM tutorial introduction to
many-to-many relationships.
:ref:`self_referential_many_to_many` - Specifics on using
many-to-many in a self-referential case.
:ref:`declarative_many_to_many` - Additional options when using
Declarative.
:ref:`association_pattern` - an alternative to
:paramref:`_orm.relationship.secondary`
when composing association
table relationships, allowing additional attributes to be
specified on the association table.
:ref:`composite_secondary_join` - a lesser-used pattern which
in some cases can enable complex :func:`_orm.relationship` SQL
conditions to be used.
.. versionadded:: 0.9.2 :paramref:`_orm.relationship.secondary`
works
more effectively when referring to a :class:`_expression.Join`
instance.
:param active_history=False:
When ``True``, indicates that the "previous" value for a
many-to-one reference should be loaded when replaced, if
not already loaded. Normally, history tracking logic for
simple many-to-ones only needs to be aware of the "new"
value in order to perform a flush. This flag is available
for applications that make use of
:func:`.attributes.get_history` which also need to know
the "previous" value of the attribute.
:param backref:
Indicates the string name of a property to be placed on the related
mapper's class that will handle this relationship in the other
direction. The other property will be created automatically
when the mappers are configured. Can also be passed as a
:func:`.backref` object to control the configuration of the
new relationship.
.. seealso::
:ref:`relationships_backref` - Introductory documentation and
examples.
:paramref:`_orm.relationship.back_populates` - alternative form
of backref specification.
:func:`.backref` - allows control over :func:`_orm.relationship`
configuration when using :paramref:`_orm.relationship.backref`.
:param back_populates:
Takes a string name and has the same meaning as
:paramref:`_orm.relationship.backref`, except the complementing
property is **not** created automatically, and instead must be
configured explicitly on the other mapper. The complementing
property should also indicate
:paramref:`_orm.relationship.back_populates` to this relationship to
ensure proper functioning.
.. seealso::
:ref:`relationships_backref` - Introductory documentation and
examples.
:paramref:`_orm.relationship.backref` - alternative form
of backref specification.
:param overlaps:
A string name or comma-delimited set of names of other relationships
on either this mapper, a descendant mapper, or a target mapper with
which this relationship may write to the same foreign keys upon
persistence. The only effect this has is to eliminate the
warning that this relationship will conflict with another upon
persistence. This is used for such relationships that are truly
capable of conflicting with each other on write, but the application
will ensure that no such conflicts occur.
.. versionadded:: 1.4
.. seealso::
:ref:`error_qzyx` - usage example
:param bake_queries=True:
Enable :ref:`lambda caching <engine_lambda_caching>` for loader
strategies, if applicable, which adds a performance gain to the
construction of SQL constructs used by loader strategies, in addition
to the usual SQL statement caching used throughout SQLAlchemy. This
parameter currently applies only to the "lazy" and "selectin" loader
strategies. There is generally no reason to set this parameter to
False.
.. versionchanged:: 1.4 Relationship loaders no longer use the
previous "baked query" system of query caching. The "lazy"
and "selectin" loaders make use of the "lambda cache" system
for the construction of SQL constructs,
as well as the usual SQL caching system that is throughout
SQLAlchemy as of the 1.4 series.
:param cascade:
A comma-separated list of cascade rules which determines how
Session operations should be "cascaded" from parent to child.
This defaults to ``False``, which means the default cascade
should be used - this default cascade is ``"save-update, merge"``.
The available cascades are ``save-update``, ``merge``,
``expunge``, ``delete``, ``delete-orphan``, and ``refresh-expire``.
An additional option, ``all`` indicates shorthand for
``"save-update, merge, refresh-expire,
expunge, delete"``, and is often used as in ``"all, delete-orphan"``
to indicate that related objects should follow along with the
parent object in all cases, and be deleted when de-associated.
.. seealso::
:ref:`unitofwork_cascades` - Full detail on each of the available
cascade options.
:ref:`tutorial_delete_cascade` - Tutorial example describing
a delete cascade.
:param cascade_backrefs=True:
A boolean value indicating if the ``save-update`` cascade should
operate along an assignment event intercepted by a backref.
When set to ``False``, the attribute managed by this relationship
will not cascade an incoming transient object into the session of a
persistent parent, if the event is received via backref.
.. deprecated:: 1.4 The
:paramref:`_orm.relationship.cascade_backrefs`
flag will default to False in all cases in SQLAlchemy 2.0.
.. seealso::
:ref:`backref_cascade` - Full discussion and examples on how
the :paramref:`_orm.relationship.cascade_backrefs` option is used.
:param collection_class:
A class or callable that returns a new list-holding object. will
be used in place of a plain list for storing elements.
.. seealso::
:ref:`custom_collections` - Introductory documentation and
examples.
:param comparator_factory:
A class which extends :class:`.RelationshipProperty.Comparator`
which provides custom SQL clause generation for comparison
operations.
.. seealso::
:class:`.PropComparator` - some detail on redefining comparators
at this level.
:ref:`custom_comparators` - Brief intro to this feature.
:param distinct_target_key=None:
Indicate if a "subquery" eager load should apply the DISTINCT
keyword to the innermost SELECT statement. When left as ``None``,
the DISTINCT keyword will be applied in those cases when the target
columns do not comprise the full primary key of the target table.
When set to ``True``, the DISTINCT keyword is applied to the
innermost SELECT unconditionally.
It may be desirable to set this flag to False when the DISTINCT is
reducing performance of the innermost subquery beyond that of what
duplicate innermost rows may be causing.
.. versionchanged:: 0.9.0 -
:paramref:`_orm.relationship.distinct_target_key` now defaults to
``None``, so that the feature enables itself automatically for
those cases where the innermost query targets a non-unique
key.
.. seealso::
:ref:`loading_toplevel` - includes an introduction to subquery
eager loading.
:param doc:
Docstring which will be applied to the resulting descriptor.
:param foreign_keys:
A list of columns which are to be used as "foreign key"
columns, or columns which refer to the value in a remote
column, within the context of this :func:`_orm.relationship`
object's :paramref:`_orm.relationship.primaryjoin` condition.
That is, if the :paramref:`_orm.relationship.primaryjoin`
condition of this :func:`_orm.relationship` is ``a.id ==
b.a_id``, and the values in ``b.a_id`` are required to be
present in ``a.id``, then the "foreign key" column of this
:func:`_orm.relationship` is ``b.a_id``.
In normal cases, the :paramref:`_orm.relationship.foreign_keys`
parameter is **not required.** :func:`_orm.relationship` will
automatically determine which columns in the
:paramref:`_orm.relationship.primaryjoin` condition are to be
considered "foreign key" columns based on those
:class:`_schema.Column` objects that specify
:class:`_schema.ForeignKey`,
or are otherwise listed as referencing columns in a
:class:`_schema.ForeignKeyConstraint` construct.
:paramref:`_orm.relationship.foreign_keys` is only needed when:
1. There is more than one way to construct a join from the local
table to the remote table, as there are multiple foreign key
references present. Setting ``foreign_keys`` will limit the
:func:`_orm.relationship`
to consider just those columns specified
here as "foreign".
2. The :class:`_schema.Table` being mapped does not actually have
:class:`_schema.ForeignKey` or
:class:`_schema.ForeignKeyConstraint`
constructs present, often because the table
was reflected from a database that does not support foreign key
reflection (MySQL MyISAM).
3. The :paramref:`_orm.relationship.primaryjoin`
argument is used to
construct a non-standard join condition, which makes use of
columns or expressions that do not normally refer to their
"parent" column, such as a join condition expressed by a
complex comparison using a SQL function.
The :func:`_orm.relationship` construct will raise informative
error messages that suggest the use of the
:paramref:`_orm.relationship.foreign_keys` parameter when
presented with an ambiguous condition. In typical cases,
if :func:`_orm.relationship` doesn't raise any exceptions, the
:paramref:`_orm.relationship.foreign_keys` parameter is usually
not needed.
:paramref:`_orm.relationship.foreign_keys` may also be passed as a
callable function which is evaluated at mapper initialization time,
and may be passed as a Python-evaluable string when using
Declarative.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
.. seealso::
:ref:`relationship_foreign_keys`
:ref:`relationship_custom_foreign`
:func:`.foreign` - allows direct annotation of the "foreign"
columns within a :paramref:`_orm.relationship.primaryjoin`
condition.
:param info: Optional data dictionary which will be populated into the
:attr:`.MapperProperty.info` attribute of this object.
:param innerjoin=False:
When ``True``, joined eager loads will use an inner join to join
against related tables instead of an outer join. The purpose
of this option is generally one of performance, as inner joins
generally perform better than outer joins.
This flag can be set to ``True`` when the relationship references an
object via many-to-one using local foreign keys that are not
nullable, or when the reference is one-to-one or a collection that
is guaranteed to have one or at least one entry.
The option supports the same "nested" and "unnested" options as
that of :paramref:`_orm.joinedload.innerjoin`. See that flag
for details on nested / unnested behaviors.
.. seealso::
:paramref:`_orm.joinedload.innerjoin` - the option as specified by
loader option, including detail on nesting behavior.
:ref:`what_kind_of_loading` - Discussion of some details of
various loader options.
:param join_depth:
When non-``None``, an integer value indicating how many levels
deep "eager" loaders should join on a self-referring or cyclical
relationship. The number counts how many times the same Mapper
shall be present in the loading condition along a particular join
branch. When left at its default of ``None``, eager loaders
will stop chaining when they encounter a the same target mapper
which is already higher up in the chain. This option applies
both to joined- and subquery- eager loaders.
.. seealso::
:ref:`self_referential_eager_loading` - Introductory documentation
and examples.
:param lazy='select': specifies
How the related items should be loaded. Default value is
``select``. Values include:
* ``select`` - items should be loaded lazily when the property is
first accessed, using a separate SELECT statement, or identity map
fetch for simple many-to-one references.
* ``immediate`` - items should be loaded as the parents are loaded,
using a separate SELECT statement, or identity map fetch for
simple many-to-one references.
* ``joined`` - items should be loaded "eagerly" in the same query as
that of the parent, using a JOIN or LEFT OUTER JOIN. Whether
the join is "outer" or not is determined by the
:paramref:`_orm.relationship.innerjoin` parameter.
* ``subquery`` - items should be loaded "eagerly" as the parents are
loaded, using one additional SQL statement, which issues a JOIN to
a subquery of the original statement, for each collection
requested.
* ``selectin`` - items should be loaded "eagerly" as the parents
are loaded, using one or more additional SQL statements, which
issues a JOIN to the immediate parent object, specifying primary
key identifiers using an IN clause.
.. versionadded:: 1.2
* ``noload`` - no loading should occur at any time. This is to
support "write-only" attributes, or attributes which are
populated in some manner specific to the application.
* ``raise`` - lazy loading is disallowed; accessing
the attribute, if its value were not already loaded via eager
loading, will raise an :exc:`~sqlalchemy.exc.InvalidRequestError`.
This strategy can be used when objects are to be detached from
their attached :class:`.Session` after they are loaded.
.. versionadded:: 1.1
* ``raise_on_sql`` - lazy loading that emits SQL is disallowed;
accessing the attribute, if its value were not already loaded via
eager loading, will raise an
:exc:`~sqlalchemy.exc.InvalidRequestError`, **if the lazy load
needs to emit SQL**. If the lazy load can pull the related value
from the identity map or determine that it should be None, the
value is loaded. This strategy can be used when objects will
remain associated with the attached :class:`.Session`, however
additional SELECT statements should be blocked.
.. versionadded:: 1.1
* ``dynamic`` - the attribute will return a pre-configured
:class:`_query.Query` object for all read
operations, onto which further filtering operations can be
applied before iterating the results. See
the section :ref:`dynamic_relationship` for more details.
* True - a synonym for 'select'
* False - a synonym for 'joined'
* None - a synonym for 'noload'
.. seealso::
:doc:`/orm/loading_relationships` - Full documentation on
relationship loader configuration.
:ref:`dynamic_relationship` - detail on the ``dynamic`` option.
:ref:`collections_noload_raiseload` - notes on "noload" and "raise"
:param load_on_pending=False:
Indicates loading behavior for transient or pending parent objects.
When set to ``True``, causes the lazy-loader to
issue a query for a parent object that is not persistent, meaning it
has never been flushed. This may take effect for a pending object
when autoflush is disabled, or for a transient object that has been
"attached" to a :class:`.Session` but is not part of its pending
collection.
The :paramref:`_orm.relationship.load_on_pending`
flag does not improve
behavior when the ORM is used normally - object references should be
constructed at the object level, not at the foreign key level, so
that they are present in an ordinary way before a flush proceeds.
This flag is not not intended for general use.
.. seealso::
:meth:`.Session.enable_relationship_loading` - this method
establishes "load on pending" behavior for the whole object, and
also allows loading on objects that remain transient or
detached.
:param order_by:
Indicates the ordering that should be applied when loading these
items. :paramref:`_orm.relationship.order_by`
is expected to refer to
one of the :class:`_schema.Column`
objects to which the target class is
mapped, or the attribute itself bound to the target class which
refers to the column.
:paramref:`_orm.relationship.order_by`
may also be passed as a callable
function which is evaluated at mapper initialization time, and may
be passed as a Python-evaluable string when using Declarative.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
:param passive_deletes=False:
Indicates loading behavior during delete operations.
A value of True indicates that unloaded child items should not
be loaded during a delete operation on the parent. Normally,
when a parent item is deleted, all child items are loaded so
that they can either be marked as deleted, or have their
foreign key to the parent set to NULL. Marking this flag as
True usually implies an ON DELETE <CASCADE|SET NULL> rule is in
place which will handle updating/deleting child rows on the
database side.
Additionally, setting the flag to the string value 'all' will
disable the "nulling out" of the child foreign keys, when the parent
object is deleted and there is no delete or delete-orphan cascade
enabled. This is typically used when a triggering or error raise
scenario is in place on the database side. Note that the foreign
key attributes on in-session child objects will not be changed after
a flush occurs so this is a very special use-case setting.
Additionally, the "nulling out" will still occur if the child
object is de-associated with the parent.
.. seealso::
:ref:`passive_deletes` - Introductory documentation
and examples.
:param passive_updates=True:
Indicates the persistence behavior to take when a referenced
primary key value changes in place, indicating that the referencing
foreign key columns will also need their value changed.
When True, it is assumed that ``ON UPDATE CASCADE`` is configured on
the foreign key in the database, and that the database will
handle propagation of an UPDATE from a source column to
dependent rows. When False, the SQLAlchemy
:func:`_orm.relationship`
construct will attempt to emit its own UPDATE statements to
modify related targets. However note that SQLAlchemy **cannot**
emit an UPDATE for more than one level of cascade. Also,
setting this flag to False is not compatible in the case where
the database is in fact enforcing referential integrity, unless
those constraints are explicitly "deferred", if the target backend
supports it.
It is highly advised that an application which is employing
mutable primary keys keeps ``passive_updates`` set to True,
and instead uses the referential integrity features of the database
itself in order to handle the change efficiently and fully.
.. seealso::
:ref:`passive_updates` - Introductory documentation and
examples.
:paramref:`.mapper.passive_updates` - a similar flag which
takes effect for joined-table inheritance mappings.
:param post_update:
This indicates that the relationship should be handled by a
second UPDATE statement after an INSERT or before a
DELETE. Currently, it also will issue an UPDATE after the
instance was UPDATEd as well, although this technically should
be improved. This flag is used to handle saving bi-directional
dependencies between two individual rows (i.e. each row
references the other), where it would otherwise be impossible to
INSERT or DELETE both rows fully since one row exists before the
other. Use this flag when a particular mapping arrangement will
incur two rows that are dependent on each other, such as a table
that has a one-to-many relationship to a set of child rows, and
also has a column that references a single child row within that
list (i.e. both tables contain a foreign key to each other). If
a flush operation returns an error that a "cyclical
dependency" was detected, this is a cue that you might want to
use :paramref:`_orm.relationship.post_update` to "break" the cycle.
.. seealso::
:ref:`post_update` - Introductory documentation and examples.
:param primaryjoin:
A SQL expression that will be used as the primary
join of the child object against the parent object, or in a
many-to-many relationship the join of the parent object to the
association table. By default, this value is computed based on the
foreign key relationships of the parent and child tables (or
association table).
:paramref:`_orm.relationship.primaryjoin` may also be passed as a
callable function which is evaluated at mapper initialization time,
and may be passed as a Python-evaluable string when using
Declarative.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
.. seealso::
:ref:`relationship_primaryjoin`
:param remote_side:
Used for self-referential relationships, indicates the column or
list of columns that form the "remote side" of the relationship.
:paramref:`_orm.relationship.remote_side` may also be passed as a
callable function which is evaluated at mapper initialization time,
and may be passed as a Python-evaluable string when using
Declarative.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
.. seealso::
:ref:`self_referential` - in-depth explanation of how
:paramref:`_orm.relationship.remote_side`
is used to configure self-referential relationships.
:func:`.remote` - an annotation function that accomplishes the
same purpose as :paramref:`_orm.relationship.remote_side`,
typically
when a custom :paramref:`_orm.relationship.primaryjoin` condition
is used.
:param query_class:
A :class:`_query.Query`
subclass that will be used internally by the
``AppenderQuery`` returned by a "dynamic" relationship, that
is, a relationship that specifies ``lazy="dynamic"`` or was
otherwise constructed using the :func:`_orm.dynamic_loader`
function.
.. seealso::
:ref:`dynamic_relationship` - Introduction to "dynamic"
relationship loaders.
:param secondaryjoin:
A SQL expression that will be used as the join of
an association table to the child object. By default, this value is
computed based on the foreign key relationships of the association
and child tables.
:paramref:`_orm.relationship.secondaryjoin` may also be passed as a
callable function which is evaluated at mapper initialization time,
and may be passed as a Python-evaluable string when using
Declarative.
.. warning:: When passed as a Python-evaluable string, the
argument is interpreted using Python's ``eval()`` function.
**DO NOT PASS UNTRUSTED INPUT TO THIS STRING**.
See :ref:`declarative_relationship_eval` for details on
declarative evaluation of :func:`_orm.relationship` arguments.
.. seealso::
:ref:`relationship_primaryjoin`
:param single_parent:
When True, installs a validator which will prevent objects
from being associated with more than one parent at a time.
This is used for many-to-one or many-to-many relationships that
should be treated either as one-to-one or one-to-many. Its usage
is optional, except for :func:`_orm.relationship` constructs which
are many-to-one or many-to-many and also
specify the ``delete-orphan`` cascade option. The
:func:`_orm.relationship` construct itself will raise an error
instructing when this option is required.
.. seealso::
:ref:`unitofwork_cascades` - includes detail on when the
:paramref:`_orm.relationship.single_parent`
flag may be appropriate.
:param uselist:
A boolean that indicates if this property should be loaded as a
list or a scalar. In most cases, this value is determined
automatically by :func:`_orm.relationship` at mapper configuration
time, based on the type and direction
of the relationship - one to many forms a list, many to one
forms a scalar, many to many is a list. If a scalar is desired
where normally a list would be present, such as a bi-directional
one-to-one relationship, set :paramref:`_orm.relationship.uselist`
to
False.
The :paramref:`_orm.relationship.uselist`
flag is also available on an
existing :func:`_orm.relationship`
construct as a read-only attribute,
which can be used to determine if this :func:`_orm.relationship`
deals
with collections or scalar attributes::
>>> User.addresses.property.uselist
True
.. seealso::
:ref:`relationships_one_to_one` - Introduction to the "one to
one" relationship pattern, which is typically when the
:paramref:`_orm.relationship.uselist` flag is needed.
:param viewonly=False:
When set to ``True``, the relationship is used only for loading
objects, and not for any persistence operation. A
:func:`_orm.relationship` which specifies
:paramref:`_orm.relationship.viewonly` can work
with a wider range of SQL operations within the
:paramref:`_orm.relationship.primaryjoin` condition, including
operations that feature the use of a variety of comparison operators
as well as SQL functions such as :func:`_expression.cast`. The
:paramref:`_orm.relationship.viewonly`
flag is also of general use when defining any kind of
:func:`_orm.relationship` that doesn't represent
the full set of related objects, to prevent modifications of the
collection from resulting in persistence operations.
When using the :paramref:`_orm.relationship.viewonly` flag in
conjunction with backrefs, the originating relationship for a
particular state change will not produce state changes within the
viewonly relationship. This is the behavior implied by
:paramref:`_orm.relationship.sync_backref` being set to False.
.. versionchanged:: 1.3.17 - the
:paramref:`_orm.relationship.sync_backref` flag is set to False
when using viewonly in conjunction with backrefs.
.. seealso::
:paramref:`_orm.relationship.sync_backref`
:param sync_backref:
A boolean that enables the events used to synchronize the in-Python
attributes when this relationship is target of either
:paramref:`_orm.relationship.backref` or
:paramref:`_orm.relationship.back_populates`.
Defaults to ``None``, which indicates that an automatic value should
be selected based on the value of the
:paramref:`_orm.relationship.viewonly` flag. When left at its
default, changes in state will be back-populated only if neither
sides of a relationship is viewonly.
.. versionadded:: 1.3.17
.. versionchanged:: 1.4 - A relationship that specifies
:paramref:`_orm.relationship.viewonly` automatically implies
that :paramref:`_orm.relationship.sync_backref` is ``False``.
.. seealso::
:paramref:`_orm.relationship.viewonly`
:param omit_join:
Allows manual control over the "selectin" automatic join
optimization. Set to ``False`` to disable the "omit join" feature
added in SQLAlchemy 1.3; or leave as ``None`` to leave automatic
optimization in place.
.. note:: This flag may only be set to ``False``. It is not
necessary to set it to ``True`` as the "omit_join" optimization is
automatically detected; if it is not detected, then the
optimization is not supported.
.. versionchanged:: 1.3.11 setting ``omit_join`` to True will now
emit a warning as this was not the intended use of this flag.
.. versionadded:: 1.3
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