Relationships API — SQLAlchemy 1.2 Documentation

Release: 1.2.19 | Release Date: April 15, 2019

# SQLAlchemy 1.2 Documentation

Functional constructs for ORM configuration.

See the SQLAlchemy object relational tutorial and mapper configuration documentation for an overview of how this module is used.

# Relationships API

`sqlalchemy.orm.` `relationship`(argument, secondary=None, primaryjoin=None, secondaryjoin=None, foreign_keys=None, uselist=None, order_by=False, backref=None, back_populates=None, post_update=False, cascade=False, extension=None, 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)

  Provide a relationship between two mapped classes.

  This corresponds to a parent-child or associative table relationship. The constructed class is an instance of `RelationshipProperty`.

  A typical `relationship()`, used in a classical mapping:

      mapper(Parent, properties={
        'children': relationship(Child)
      })

  Some arguments accepted by `relationship()` optionally accept a callable function, which when called produces the desired value. The callable is invoked by the parent `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 Declarative extension, the Declarative initializer allows string arguments to be passed to `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 `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")

  See also

  Relationship Configuration - Full introductory and reference documentation for `relationship()`.

  Building a Relationship - ORM tutorial introduction.

  Parameters

    - argument –

      a mapped class, or actual `Mapper` instance, representing the target of the relationship.

      `argument` 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.

      See also

      Configuring Relationships - further detail on relationship configuration when using Declarative.

    - secondary –

      for a many-to-many relationship, specifies the intermediary table, and is typically an instance of `Table`. In less common circumstances, the argument may also be specified as an `Alias` construct, or even a `Join` construct.

      `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 `Table` that is present in the `MetaData` collection associated with the parent-mapped `Table`.

      The `secondary` keyword argument is typically applied in the case where the intermediary `Table` is not otherwise expressed in any direct class mapping. If the “secondary” table is also explicitly mapped elsewhere (e.g. as in Association Object), one should consider applying the `viewonly` flag so that this `relationship()` is not used for persistence operations which may conflict with those of the association object pattern.

      See also

      Many To Many - Reference example of “many to many”.

      Building a Many To Many Relationship - ORM tutorial introduction to many-to-many relationships.

      Self-Referential Many-to-Many Relationship - Specifics on using many-to-many in a self-referential case.

      Configuring Many-to-Many Relationships - Additional options when using Declarative.

      Association Object - an alternative to `secondary` when composing association table relationships, allowing additional attributes to be specified on the association table.

      Composite “Secondary” Joins - a lesser-used pattern which in some cases can enable complex `relationship()` SQL conditions to be used.

      New in version 0.9.2: `secondary` works more effectively when referring to a `Join` instance.

    - 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 `attributes.get_history()` which also need to know the “previous” value of the attribute.

    - 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 `backref()` object to control the configuration of the new relationship.

      See also

      Linking Relationships with Backref - Introductory documentation and examples.

      `back_populates` - alternative form of backref specification.

      `backref()` - allows control over `relationship()` configuration when using `backref`.

    - back_populates –

      Takes a string name and has the same meaning as `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 `back_populates` to this relationship to ensure proper functioning.

      See also

      Linking Relationships with Backref - Introductory documentation and examples.

      `backref` - alternative form of backref specification.

    - bake_queries=True –

      Use the `BakedQuery` cache to cache the construction of SQL used in lazy loads. True by default. Set to False if the join condition of the relationship has unusual features that might not respond well to statement caching.

      Changed in version 1.2: “Baked” loading is the default implementation for the “select”, a.k.a. “lazy” loading strategy for relationships.

      New in version 1.0.0.

      See also

      Baked Queries

    - 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.

      See also

      Cascades - Full detail on each of the available cascade options.

      Configuring delete/delete-orphan Cascade - Tutorial example describing a delete cascade.

    - 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.

      See also

      Controlling Cascade on Backrefs - Full discussion and examples on how the `cascade_backrefs` option is used.

    - 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.

      See also

      Customizing Collection Access - Introductory documentation and examples.

    - comparator_factory –

      a class which extends `RelationshipProperty.Comparator` which provides custom SQL clause generation for comparison operations.

      See also

      `PropComparator` - some detail on redefining comparators at this level.

      Operator Customization - Brief intro to this feature.

    - 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.

      Changed in version 0.9.0: - `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.

      See also

      Relationship Loading Techniques - includes an introduction to subquery eager loading.

    - doc – docstring which will be applied to the resulting descriptor.

    - extension –

      an `AttributeExtension` instance, or list of extensions, which will be prepended to the list of attribute listeners for the resulting descriptor placed on the class.

      > Deprecated since version 0.7: `AttributeExtension` is deprecated in favor of the `AttributeEvents` listener interface. The `relationship.extension` parameter will be removed in a future release.

    - 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 `relationship()` object’s `primaryjoin` condition. That is, if the `primaryjoin` condition of this `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 `relationship()` is `b.a_id`.

      In normal cases, the `foreign_keys` parameter is not required. `relationship()` will automatically determine which columns in the `primaryjoin` condition are to be considered “foreign key” columns based on those `Column` objects that specify `ForeignKey`, or are otherwise listed as referencing columns in a `ForeignKeyConstraint` construct. `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 `relationship()` to consider just those columns specified here as “foreign”.

      > 2. The `Table` being mapped does not actually have `ForeignKey` or `ForeignKeyConstraint` constructs present, often because the table was reflected from a database that does not support foreign key reflection (MySQL MyISAM).

      > 3. The `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 `relationship()` construct will raise informative error messages that suggest the use of the `foreign_keys` parameter when presented with an ambiguous condition. In typical cases, if `relationship()` doesn’t raise any exceptions, the `foreign_keys` parameter is usually not needed.

      `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.

      See also

      Handling Multiple Join Paths

      Creating Custom Foreign Conditions

      `foreign()` - allows direct annotation of the “foreign” columns within a `primaryjoin` condition.

    - info – Optional data dictionary which will be populated into the `MapperProperty.info` attribute of this object.

    - 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 `joinedload.innerjoin`. See that flag for details on nested / unnested behaviors.

      See also

      `joinedload.innerjoin` - the option as specified by loader option, including detail on nesting behavior.

      What Kind of Loading to Use ? - Discussion of some details of various loader options.

    - 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.

      See also

      Configuring Self-Referential Eager Loading - Introductory documentation and examples.

    - 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 `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.

        New in version 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 `InvalidRequestError`. This strategy can be used when objects are to be detached from their attached `Session` after they are loaded.

        New in version 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 `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 `Session`, however additional SELECT statements should be blocked.

        New in version 1.1.

      - `dynamic` - the attribute will return a pre-configured `Query` object for all read operations, onto which further filtering operations can be applied before iterating the results. See the section Dynamic Relationship Loaders for more details.

      - True - a synonym for ‘select’

      - False - a synonym for ‘joined’

      - None - a synonym for ‘noload’

      See also

      Relationship Loading Techniques - Full documentation on relationship loader configuration.

      Dynamic Relationship Loaders - detail on the `dynamic` option.

      Setting Noload, RaiseLoad - notes on “noload” and “raise”

    - 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 `Session` but is not part of its pending collection.

      The `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.

      See also

      `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.

    - order_by –

      indicates the ordering that should be applied when loading these items. `order_by` is expected to refer to one of the `Column` objects to which the target class is mapped, or the attribute itself bound to the target class which refers to the column.

      `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.

    - 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.

      See also

      Using Passive Deletes - Introductory documentation and examples.

    - 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 `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.

      See also

      Mutable Primary Keys / Update Cascades - Introductory documentation and examples.

      `mapper.passive_updates` - a similar flag which takes effect for joined-table inheritance mappings.

    - 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 `post_update` to “break” the cycle.

      See also

      Rows that point to themselves / Mutually Dependent Rows - Introductory documentation and examples.

    - primaryjoin –

      a SQL expression that will be used as the primary join of this child object against the parent object, or in a many-to-many relationship the join of the primary 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).

      `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.

      See also

      Specifying Alternate Join Conditions

    - remote_side –

      used for self-referential relationships, indicates the column or list of columns that form the “remote side” of the relationship.

      `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.

      See also

      Adjacency List Relationships - in-depth explanation of how `remote_side` is used to configure self-referential relationships.

      `remote()` - an annotation function that accomplishes the same purpose as `remote_side`, typically when a custom `primaryjoin` condition is used.

    - query_class –

      a `Query` subclass that will be used as the base of the “appender query” returned by a “dynamic” relationship, that is, a relationship that specifies `lazy="dynamic"` or was otherwise constructed using the `orm.dynamic_loader()` function.

      See also

      Dynamic Relationship Loaders - Introduction to “dynamic” relationship loaders.

    - 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.

      `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.

      See also

      Specifying Alternate Join Conditions

    - 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 `relationship()` constructs which are many-to-one or many-to-many and also specify the `delete-orphan` cascade option. The `relationship()` construct itself will raise an error instructing when this option is required.

      See also

      Cascades - includes detail on when the `single_parent` flag may be appropriate.

    - 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 `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 `uselist` to False.

      The `uselist` flag is also available on an existing `relationship()` construct as a read-only attribute, which can be used to determine if this `relationship()` deals with collections or scalar attributes:

          >>> User.addresses.property.uselist
          True

      See also

      One To One - Introduction to the “one to one” relationship pattern, which is typically when the `uselist` flag is needed.

    - viewonly=False – when set to True, the relationship is used only for loading objects, and not for any persistence operation. A `relationship()` which specifies `viewonly` can work with a wider range of SQL operations within the `primaryjoin` condition, including operations that feature the use of a variety of comparison operators as well as SQL functions such as `cast()`. The `viewonly` flag is also of general use when defining any kind of `relationship()` that doesn’t represent the full set of related objects, to prevent modifications of the collection from resulting in persistence operations.

`sqlalchemy.orm.` `backref`(name, **kwargs)

  Create a back reference with explicit keyword arguments, which are the same arguments one can send to `relationship()`.

  Used with the `backref` keyword argument to `relationship()` in place of a string argument, e.g.:

      'items':relationship(
          SomeItem, backref=backref('parent', lazy='subquery'))

  See also

  Linking Relationships with Backref

`sqlalchemy.orm.` `relation`(*arg, **kw)

  A synonym for `relationship()`.

`sqlalchemy.orm.` `dynamic_loader`(argument, **kw)

  Construct a dynamically-loading mapper property.

  This is essentially the same as using the `lazy='dynamic'` argument with `relationship()`:

      dynamic_loader(SomeClass)

      # is the same as

      relationship(SomeClass, lazy="dynamic")

  See the section Dynamic Relationship Loaders for more details on dynamic loading.

`sqlalchemy.orm.` `foreign`(expr)

  Annotate a portion of a primaryjoin expression with a ‘foreign’ annotation.

  See the section Creating Custom Foreign Conditions for a description of use.

  See also

  Creating Custom Foreign Conditions

  `remote()`

`sqlalchemy.orm.` `remote`(expr)

  Annotate a portion of a primaryjoin expression with a ‘remote’ annotation.

  See the section Creating Custom Foreign Conditions for a description of use.

  See also

  Creating Custom Foreign Conditions

  `foreign()`