sqlalchemy.orm.properties.RelationshipProperty

A mapped class, or actual _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.

For a many-to-many relationship, specifies the intermediary table, and is typically an instance of _schema.Table. In less common circumstances, the argument may also be specified as an _expression.Alias construct, or even a _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 _schema.Table that is present in the _schema.MetaData collection associated with the parent-mapped _schema.Table.

The :paramref:`_orm.relationship.secondary` keyword argument is typically applied in the case where the intermediary _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 _orm.relationship is not used for persistence operations which may conflict with those of the association object pattern.

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.

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.

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 _orm.relationship object's :paramref:`_orm.relationship.primaryjoin` condition. That is, if the :paramref:`_orm.relationship.primaryjoin` condition of this _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 _orm.relationship is b.a_id.

In normal cases, the :paramref:`_orm.relationship.foreign_keys` parameter is not required. _orm.relationship will automatically determine which columns in the :paramref:`_orm.relationship.primaryjoin` condition are to be considered "foreign key" columns based on those _schema.Column objects that specify _schema.ForeignKey, or are otherwise listed as referencing columns in a _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 _orm.relationship to consider just those columns specified here as "foreign".
2. The _schema.Table being mapped does not actually have _schema.ForeignKey or _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 _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 _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.

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 _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 _orm.relationship construct as a read-only attribute, which can be used to determine if this _orm.relationship deals with collections or scalar attributes:

>>> User.addresses.property.uselist
True

Indicates the ordering that should be applied when loading these items. :paramref:`_orm.relationship.order_by` is expected to refer to one of the _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.

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.

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.

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.

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.

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.

A class or callable that returns a new list-holding object. will be used in place of a plain list for storing elements.

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.

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.

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

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

A _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 _orm.dynamic_loader function.

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.

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.

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.

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.

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.

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.

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.

  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 ~sqlalchemy.exc.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 ~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 .Session, however additional SELECT statements should be blocked.

  New in version 1.1.

• dynamic - the attribute will return a pre-configured _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'

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 :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.

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.

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 _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.

When set to True, the relationship is used only for loading objects, and not for any persistence operation. A _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 _expression.cast. The :paramref:`_orm.relationship.viewonly` flag is also of general use when defining any kind of _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.