SQLAlchemy 2.0 Documentation
SQLAlchemy ORM
- ORM Quick Start
- ORM Mapped Class Configuration
- Relationship Configuration
- ORM Querying Guide
- Using the Session
- Events and Internals
- ORM Extensions
- ORM Examples
Project Versions
Relationships API¶
Object Name | Description |
---|---|
backref(name, **kwargs) |
When using the |
dynamic_loader([argument], **kw) |
Construct a dynamically-loading mapper property. |
foreign(expr) |
Annotate a portion of a primaryjoin expression with a ‘foreign’ annotation. |
relationship([argument, secondary], *, [uselist, collection_class, primaryjoin, secondaryjoin, back_populates, order_by, backref, overlaps, post_update, cascade, viewonly, init, repr, default, default_factory, compare, kw_only, lazy, passive_deletes, passive_updates, active_history, enable_typechecks, foreign_keys, remote_side, join_depth, comparator_factory, single_parent, innerjoin, distinct_target_key, load_on_pending, query_class, info, omit_join, sync_backref], **kw) |
Provide a relationship between two mapped classes. |
remote(expr) |
Annotate a portion of a primaryjoin expression with a ‘remote’ annotation. |
- function sqlalchemy.orm.relationship(argument: _RelationshipArgumentType[Any] | None = None, secondary: _RelationshipSecondaryArgument | None = None, *, uselist: bool | None = None, collection_class: Type[Collection[Any]] | Callable[[], Collection[Any]] | None = None, primaryjoin: _RelationshipJoinConditionArgument | None = None, secondaryjoin: _RelationshipJoinConditionArgument | None = None, back_populates: str | None = None, order_by: _ORMOrderByArgument = False, backref: ORMBackrefArgument | None = None, overlaps: str | None = None, post_update: bool = False, cascade: str = 'save-update, merge', viewonly: bool = False, init: _NoArg | bool = _NoArg.NO_ARG, repr: _NoArg | bool = _NoArg.NO_ARG, default: _NoArg | _T = _NoArg.NO_ARG, default_factory: _NoArg | Callable[[], _T] = _NoArg.NO_ARG, compare: _NoArg | bool = _NoArg.NO_ARG, kw_only: _NoArg | bool = _NoArg.NO_ARG, lazy: _LazyLoadArgumentType = 'select', passive_deletes: Literal['all'] | bool = False, passive_updates: bool = True, active_history: bool = False, enable_typechecks: bool = True, foreign_keys: _ORMColCollectionArgument | None = None, remote_side: _ORMColCollectionArgument | None = None, join_depth: int | None = None, comparator_factory: Type[RelationshipProperty.Comparator[Any]] | None = None, single_parent: bool = False, innerjoin: bool = False, distinct_target_key: bool | None = None, load_on_pending: bool = False, query_class: Type[Query[Any]] | None = None, info: _InfoType | None = None, omit_join: Literal[None, False] = None, sync_backref: bool | None = None, **kw: Any) → _RelationshipDeclared[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
Relationship
.See also
Working with ORM Related Objects - tutorial introduction to
relationship()
in the SQLAlchemy Unified TutorialRelationship Configuration - 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 orMapper
when called, and finally a string name for the class, which will be resolved from theregistry
in use in order to locate the class, e.g.:class SomeClass(Base): # ... related = relationship("RelatedClass")
The
relationship.argument
may also be omitted from therelationship()
construct entirely, and instead placed inside aMapped
annotation on the left side, which should include a Python collection type if the relationship is expected to be a collection, such as:class SomeClass(Base): # ... related_items: Mapped[List["RelatedItem"]] = relationship()
Or for a many-to-one or one-to-one relationship:
class SomeClass(Base): # ... related_item: Mapped["RelatedItem"] = 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
Table
. In less common circumstances, the argument may also be specified as anAlias
construct, or even aJoin
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 aTable
that is present in theMetaData
collection associated with the parent-mappedTable
.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 ofrelationship()
arguments.The
relationship.secondary
keyword argument is typically applied in the case where the intermediaryTable
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 therelationship.viewonly
flag so that thisrelationship()
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”.
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
relationship.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.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 ofget_history()
which also need to know the “previous” value of the attribute.backref¶ –
A reference to a string relationship name, or a
backref()
construct, which will be used to automatically generate a newrelationship()
on the related class, which then refers to this one using a bi-directionalrelationship.back_populates
configuration.In modern Python, explicit use of
relationship()
withrelationship.back_populates
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
Using the legacy ‘backref’ relationship parameter - notes on using
relationship.backref
Working with ORM Related Objects - in the SQLAlchemy Unified Tutorial, presents an overview of bi-directional relationship configuration and behaviors using
relationship.back_populates
backref()
- allows control overrelationship()
configuration when usingrelationship.backref
.back_populates¶ –
Indicates the name of a
relationship()
on the related class that will be synchronized with this one. It is usually expected that therelationship()
on the related class also refer to this one. This allows objects on both sides of eachrelationship()
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
Working with ORM Related Objects - in the SQLAlchemy Unified Tutorial, presents an overview of bi-directional relationship configuration and behaviors.
Basic Relationship Patterns - includes many examples of
relationship.back_populates
.relationship.backref
- legacy form which allows more succinct configuration, but does not support explicit typingoverlaps¶ –
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
, andrefresh-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.
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
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 toTrue
, 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.
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
relationship()
object’srelationship.primaryjoin
condition. That is, if therelationship.primaryjoin
condition of thisrelationship()
isa.id == b.a_id
, and the values inb.a_id
are required to be present ina.id
, then the “foreign key” column of thisrelationship()
isb.a_id
.In normal cases, the
relationship.foreign_keys
parameter is not required.relationship()
will automatically determine which columns in therelationship.primaryjoin
condition are to be considered “foreign key” columns based on thoseColumn
objects that specifyForeignKey
, or are otherwise listed as referencing columns in aForeignKeyConstraint
construct.relationship.foreign_keys
is only needed when: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 therelationship()
to consider just those columns specified here as “foreign”.The
Table
being mapped does not actually haveForeignKey
orForeignKeyConstraint
constructs present, often because the table was reflected from a database that does not support foreign key reflection (MySQL MyISAM).The
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
relationship()
construct will raise informative error messages that suggest the use of therelationship.foreign_keys
parameter when presented with an ambiguous condition. In typical cases, ifrelationship()
doesn’t raise any exceptions, therelationship.foreign_keys
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
eval()
function. DO NOT PASS UNTRUSTED INPUT TO THIS STRING. See Evaluation of relationship arguments for details on declarative evaluation ofrelationship()
arguments.See also
Creating Custom Foreign Conditions
foreign()
- allows direct annotation of the “foreign” columns within arelationship.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 ofNone
, 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 therelationship.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. Thenoload
strategy is not recommended for general use. For a general use “never load” approach, see Write Only Relationshipsraise
- lazy loading is disallowed; accessing the attribute, if its value were not already loaded via eager loading, will raise anInvalidRequestError
. This strategy can be used when objects are to be detached from their attachedSession
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 anInvalidRequestError
, 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 attachedSession
, however additional SELECT statements should be blocked.write_only
- the attribute will be configured with a special “virtual collection” that may receiveWriteOnlyCollection.add()
andWriteOnlyCollection.remove()
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 asWriteOnlyCollection.select()
,WriteOnlyCollection.insert()
,WriteOnlyCollection.update()
andWriteOnlyCollection.delete()
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 theWriteOnlyMapped
annotation is provided on the left hand side within a Declarative mapping. See the section Write Only Relationships for examples.New in version 2.0.
See also
dynamic
- the attribute will return a pre-configuredQuery
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 theDynamicMapped
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 Write Only Relationships.
See also
Dynamic Relationship Loaders - in the ORM Querying Guide
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
Relationship Loading Techniques - 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 aSession
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
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.
relationship.order_by
is expected to refer to one of theColumn
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 Evaluation of relationship arguments for details on declarative evaluation ofrelationship()
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
Using foreign key ON DELETE cascade with ORM relationships - 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 SQLAlchemyrelationship()
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. 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
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 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).
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 Evaluation of relationship arguments for details on declarative evaluation ofrelationship()
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 Evaluation of relationship arguments for details on declarative evaluation ofrelationship()
arguments.See also
Adjacency List Relationships - in-depth explanation of how
relationship.remote_side
is used to configure self-referential relationships.remote()
- an annotation function that accomplishes the same purpose asrelationship.remote_side
, typically when a customrelationship.primaryjoin
condition is used.query_class¶ –
A
Query
subclass that will be used internally by theAppenderQuery
returned by a “dynamic” relationship, that is, a relationship that specifieslazy="dynamic"
or was otherwise constructed using thedynamic_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.
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 Evaluation of relationship arguments for details on declarative evaluation ofrelationship()
arguments.See also
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 thedelete-orphan
cascade option. Therelationship()
construct itself will raise an error instructing when this option is required.See also
Cascades - 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
relationship()
at mapper configuration time. When using explicitMapped
annotations,relationship.uselist
may be derived from the whether or not the annotation withinMapped
contains a collection class. Otherwise,relationship.uselist
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 appropriateMapped
annotation or setrelationship.uselist
to False.The
relationship.uselist
flag is also available on an existingrelationship()
construct as a read-only attribute, which can be used to determine if thisrelationship()
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 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. Arelationship()
which specifiesrelationship.viewonly
can work with a wider range of SQL operations within therelationship.primaryjoin
condition, including operations that feature the use of a variety of comparison operators as well as SQL functions such ascast()
. Therelationship.viewonly
flag is also of general use when defining any kind ofrelationship()
that doesn’t represent the full set of related objects, to prevent modifications of the collection from resulting in persistence operations.See also
Notes on using the viewonly relationship parameter - more details on best practices when using
relationship.viewonly
.sync_backref¶ –
A boolean that enables the events used to synchronize the in-Python attributes when this relationship is target of either
relationship.backref
orrelationship.back_populates
.Defaults to
None
, which indicates that an automatic value should be selected based on the value of therelationship.viewonly
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 thatrelationship.sync_backref
isFalse
.See also
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 asNone
to leave automatic optimization in place.Note
This flag may only be set to
False
. It is not necessary to set it toTrue
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 Declarative Dataclass Mapping, 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 Declarative Dataclass Mapping, 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 Declarative Dataclass Mapping, 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 newrelationship()
is generated.E.g.:
'items':relationship( SomeItem, backref=backref('parent', lazy='subquery'))
The
relationship.backref
parameter is generally considered to be legacy; for modern applications, using explicitrelationship()
constructs linked together using therelationship.back_populates
parameter should be preferred.See also
Using the legacy ‘backref’ relationship parameter - background on backrefs
- function sqlalchemy.orm.dynamic_loader(argument: _RelationshipArgumentType[Any] | None = None, **kw: Any) → RelationshipProperty[Any]¶
Construct a dynamically-loading mapper property.
This is essentially the same as using the
lazy='dynamic'
argument withrelationship()
: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 Creating Custom Foreign Conditions for a description of use.
- 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.
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