Relationships API

function sqlalchemy.orm.relationship(argument: Optional[_RelationshipArgumentType[Any]] = None, secondary: Optional[_RelationshipSecondaryArgument] = None, *, uselist: Optional[bool] = None, collection_class: Optional[Union[Type[Collection[Any]], Callable[[], Collection[Any]]]] = None, primaryjoin: Optional[_RelationshipJoinConditionArgument] = None, secondaryjoin: Optional[_RelationshipJoinConditionArgument] = None, back_populates: Optional[str] = None, order_by: _ORMOrderByArgument = False, backref: Optional[ORMBackrefArgument] = None, overlaps: Optional[str] = None, post_update: bool = False, cascade: str = ‘save-update, merge’, viewonly: bool = False, init: Union[_NoArg, bool] = _NoArg.NO_ARG, repr: Union[_NoArg, bool] = _NoArg.NO_ARG, default: Union[_NoArg, _T] = _NoArg.NO_ARG, default_factory: Union[_NoArg, Callable[[], _T]] = _NoArg.NO_ARG, compare: Union[_NoArg, bool] = _NoArg.NO_ARG, kw_only: Union[_NoArg, bool] = _NoArg.NO_ARG, lazy: _LazyLoadArgumentType = ‘select’, passive_deletes: Union[Literal[‘all’], bool] = False, passive_updates: bool = True, active_history: bool = False, enable_typechecks: bool = True, foreign_keys: Optional[_ORMColCollectionArgument] = None, remote_side: Optional[_ORMColCollectionArgument] = None, join_depth: Optional[int] = None, comparator_factory: Optional[Type[RelationshipProperty.Comparator[Any]]] = None, single_parent: bool = False, innerjoin: bool = False, distinct_target_key: Optional[bool] = None, load_on_pending: bool = False, query_class: Optional[Type[[Any]]] = None, info: Optional[_InfoType] = None, omit_join: Literal[None, False] = None, sync_backref: Optional[bool] = None, **kw: Any) → Relationship[Any]

Provide a relationship between two mapped classes.

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

See also

Working with Related Objects - tutorial introduction to in the SQLAlchemy Unified Tutorial

- narrative documentation

Parameters:
- argument –
  
  This parameter refers to the class that is to be related. It accepts several forms, including a direct reference to the target class itself, the Mapper instance for the target class, a Python callable / lambda that will return a reference to the class or when called, and finally a string name for the class, which will be resolved from the registry in use in order to locate the class, e.g.:
  
  The may also be omitted from the relationship() construct entirely, and instead placed inside a annotation on the left side, which should include a Python collection type if the relationship is expected to be a collection, such as:
```
    # ...
    related_items: Mapped[List["RelatedItem"]] = relationship()
```
  Or for a many-to-one or one-to-one relationship:
  
  See also
  
  Defining Mapped Properties with Declarative - 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 . In less common circumstances, the argument may also be specified as an Alias construct, or even a construct.
  
  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 that is present in the MetaData collection associated with the parent-mapped .
  
  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 Evaluation of relationship arguments for details on declarative evaluation of arguments.
  
  The relationship.secondary keyword argument is typically applied in the case where the intermediary 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 flag so that this relationship() is not used for persistence operations which may conflict with those of the association object pattern.
  
  See also
  
  - Reference example of “many to many”.
  
  Self-Referential Many-to-Many Relationship - Specifics on using many-to-many in a self-referential case.
  
  - Additional options when using Declarative.
  
  Association Object - an alternative to 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 SQL conditions to be used.
  
  New in version 0.9.2: relationship.secondary works more effectively when referring to a 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 get_history() which also need to know the “previous” value of the attribute.
- backref –
  
  A reference to a string relationship name, or a construct, which will be used to automatically generate a new relationship() on the related class, which then refers to this one using a bi-directional configuration.
  
  In modern Python, explicit use of relationship() with should be preferred, as it is more robust in terms of mapper configuration as well as more conceptually straightforward. It also integrates with new PEP 484 typing features introduced in SQLAlchemy 2.0 which is not possible with dynamically generated attributes.
  
  See also
  
  - notes on using relationship.backref
  
  - in the SQLAlchemy Unified Tutorial, presents an overview of bi-directional relationship configuration and behaviors using
  
  backref() - allows control over configuration when using relationship.backref.
- back_populates –
  
  Indicates the name of a on the related class that will be synchronized with this one. It is usually expected that the relationship() on the related class also refer to this one. This allows objects on both sides of each to synchronize in-Python state changes and also provides directives to the unit of work flush process how changes along these relationships should be persisted.
  
  See also
  
  - in the SQLAlchemy Unified Tutorial, presents an overview of bi-directional relationship configuration and behaviors.
  
  - includes many examples of relationship.back_populates.
  
  - legacy form which allows more succinct configuration, but does not support explicit typing
- 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.
  
  New in version 1.4.
  
  See also
  
  relationship X will copy column Q to column P, which conflicts with relationship(s): ‘Y’ - usage example
- 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
  
  - Full detail on each of the available cascade options.
- cascade_backrefs=False –
  
  Legacy; this flag is always False.
  
  Changed in version 2.0: “cascade_backrefs” functionality has been removed.
- 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 Comparator which provides custom SQL clause generation for comparison operations.
  
  See also
  
  - 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: - 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.
- 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 object’s relationship.primaryjoin condition. That is, if the 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 is b.a_id.
  
  The relationship() construct will raise informative error messages that suggest the use of the parameter when presented with an ambiguous condition. In typical cases, if relationship() doesn’t raise any exceptions, the parameter is usually not needed.
  
  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 function. DO NOT PASS UNTRUSTED INPUT TO THIS STRING. See for details on declarative evaluation of relationship() arguments.
  
  See also
  
  Creating Custom Foreign Conditions
  
  - allows direct annotation of the “foreign” columns within a relationship.primaryjoin condition.
- info – Optional data dictionary which will be populated into the 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
  
  - 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
  
  - 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 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.
  - noload - no loading should occur at any time. The related collection will remain empty. The noload strategy is not recommended for general use. For a general use “never load” approach, see
  - 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 after they are loaded.
  - 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 , however additional SELECT statements should be blocked.
    
    New in version 1.1.
  - write_only - the attribute will be configured with a special “virtual collection” that may receive WriteOnlyCollection.add() and commands to add or remove individual objects, but will not under any circumstances load or iterate the full set of objects from the database directly. Instead, methods such as WriteOnlyCollection.select(), , WriteOnlyCollection.update() and are provided which generate SQL constructs that may be used to load and modify rows in bulk. Used for large collections that are never appropriate to load at once into memory.
    
    The write_only loader style is configured automatically when the WriteOnlyMapped annotation is provided on the left hand side within a Declarative mapping. See the section for examples.
    
    New in version 2.0.
    
    See also
    
    Write Only Relationships - in the
  - 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.
    
    The dynamic loader style is configured automatically when the annotation is provided on the left hand side within a Declarative mapping. See the section Dynamic Relationship Loaders for examples.
    
    Legacy Feature
    
    The “dynamic” lazy loader strategy is the legacy form of what is now the “write_only” strategy described in the section .
    
    See also
    
    Dynamic Relationship Loaders - in the
    
    Write Only Relationships - more generally useful approach for large collections that should not fully load into memory
  - True - a synonym for ‘select’
  - False - a synonym for ‘joined’
  - None - a synonym for ‘noload’
  See also
  
  - Full documentation on relationship loader configuration in the ORM Querying Guide.
- 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 but is not part of its pending collection.
  
  The 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.
  
  See also
  
  - 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. relationship.order_by is expected to refer to one of the objects to which the target class is mapped, or the attribute itself bound to the target class which refers to the column.
  
  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 for details on declarative evaluation of relationship() arguments.
- 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
  
  - Introductory documentation and examples.
- post_update –
  
  This indicates that the relationship should be handled by a second UPDATE statement after an INSERT or before a DELETE. 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 relationship.post_update to “break” the cycle.
  
  See also
  
  - Introductory documentation and examples.
- primaryjoin –
  
  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 for details on declarative evaluation of relationship() arguments.
  
  See also
- 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.
  
  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 for details on declarative evaluation of relationship() arguments.
  
  See also
  
  - in-depth explanation of how relationship.remote_side is used to configure self-referential relationships.
  
  - an annotation function that accomplishes the same purpose as relationship.remote_side, typically when a custom condition is used.
- query_class –
  
  A Query subclass that will be used internally by the returned by a “dynamic” relationship, that is, a relationship that specifies lazy="dynamic" or was otherwise constructed using the 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.
  
  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 Evaluation of relationship arguments for details on declarative evaluation of arguments.
  
  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 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
  
  - includes detail on when the relationship.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 at mapper configuration time. When using explicit Mapped annotations, may be derived from the whether or not the annotation within Mapped contains a collection class. Otherwise, may be derived from 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, use an appropriate Mapped annotation or set to False.
  
  The relationship.uselist flag is also available on an existing 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
  
  - Introduction to the “one to one” relationship pattern, which is typically when an alternate setting for relationship.uselist is involved.
- viewonly=False –
  
  When set to True, the relationship is used only for loading objects, and not for any persistence operation. A which specifies relationship.viewonly can work with a wider range of SQL operations within the condition, including operations that feature the use of a variety of comparison operators as well as SQL functions such as cast(). The 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.
  
  When using the 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 relationship.sync_backref being set to False.
  
  Changed in version 1.3.17: - the flag is set to False when using viewonly in conjunction with backrefs.
  
  See also
  
  relationship.sync_backref
- sync_backref –
  
  A boolean that enables the events used to synchronize the in-Python attributes when this relationship is target of either or relationship.back_populates.
  
  Defaults to None, which indicates that an automatic value should be selected based on the value of the flag. When left at its default, changes in state will be back-populated only if neither sides of a relationship is viewonly.
  
  New in version 1.3.17.
  
  Changed in version 1.4: - A relationship that specifies relationship.viewonly automatically implies that is False.
  
  See also
  
  relationship.viewonly
- 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.
  
  Changed in version 1.3.11: setting omit_join to True will now emit a warning as this was not the intended use of this flag.
  
  New in version 1.3.
- init – Specific to , specifies if the mapped attribute should be part of the __init__() method as generated by the dataclass process.
- repr – Specific to Declarative Dataclass Mapping, specifies if the mapped attribute should be part of the __repr__() method as generated by the dataclass process.
- default_factory – Specific to , specifies a default-value generation function that will take place as part of the __init__() method as generated by the dataclass process.
- compare –
  
  Specific to Declarative Dataclass Mapping, indicates if this field should be included in comparison operations when generating the __eq__() and __ne__() methods for the mapped class.
  
  New in version 2.0.0b4.
- kw_only – Specific to , indicates if this field should be marked as keyword-only when generating the __init__().

function sqlalchemy.orm.backref(name: str, **kwargs: Any) → ORMBackrefArgument

When using the relationship.backref parameter, provides specific parameters to be used when the new is generated.

E.g.:

The relationship.backref parameter is generally considered to be legacy; for modern applications, using explicit constructs linked together using the relationship.back_populates parameter should be preferred.

See also

- background on backrefs

function sqlalchemy.orm.dynamic_loader(argument: Optional[_RelationshipArgumentType[Any]] = None, **kw: Any) → RelationshipProperty[Any]

Construct a dynamically-loading mapper property.

This is essentially the same as using the lazy='dynamic' argument with :

dynamic_loader(SomeClass)
# is the same as
relationship(SomeClass, lazy="dynamic")

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

function sqlalchemy.orm.foreign(expr: _CEA) → _CEA

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

See the section for a description of use.

See also

Creating Custom Foreign Conditions

function sqlalchemy.orm.remote(expr: _CEA) → _CEA

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