SQLAlchemy 1.1 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=True, 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, strategy_class=None, _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 parentMapper
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 ifChild
is declared belowParent
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 to import related classes at all into the local module space: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")
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Relationship Configuration - Full introductory and reference documentation for
relationship()
.Building a Relationship - ORM tutorial introduction.
매개 변수: - 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.더 보기
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 anAlias
construct, or even aJoin
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 aTable
that is present in theMetaData
collection associated with the parent-mappedTable
.The
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 theviewonly
flag so that thisrelationship()
is not used for persistence operations which may conflict with those of the association object pattern.더 보기
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. - 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 ofattributes.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.더 보기
Linking Relationships with Backref - Introductory documentation and examples.
back_populates
- alternative form of backref specification.backref()
- allows control overrelationship()
configuration when usingbackref
. - 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 indicateback_populates
to this relationship to ensure proper functioning.더 보기
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, when thebake_lazy_loaders()
function has first been called. Defaults to True and is intended to provide an “opt out” flag per-relationship when the baked query cache system is in use.경고
This flag only has an effect when the application-wide
bake_lazy_loaders()
function has been called. It defaults to True so is an “opt out” flag.Setting this flag to False when baked queries are otherwise in use might be to reduce ORM memory use for this
relationship()
, or to work around unresolved stability issues observed within the baked query cache system.버전 1.0.0에 추가.
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- 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.더 보기
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 toFalse
, 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.더 보기
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.
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Customizing Collection Access - Introductory documentation and examples.
- comparator_factory¶ –
a class which extends
RelationshipProperty.Comparator
which provides custom SQL clause generation for comparison operations.더 보기
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.
버전 0.8.3에 추가: -
distinct_target_key
allows the subquery eager loader to apply a DISTINCT modifier to the innermost SELECT.버전 0.9.0으로 변경: -
distinct_target_key
now defaults toNone
, so that the feature enables itself automatically for those cases where the innermost query targets a non-unique key.더 보기
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.버전 0.7 폐지: Please see
AttributeEvents
. - 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’sprimaryjoin
condition. That is, if theprimaryjoin
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
foreign_keys
parameter is not required.relationship()
will automatically determine which columns in theprimaryjoin
conditition are to be considered “foreign key” columns based on thoseColumn
objects that specifyForeignKey
, or are otherwise listed as referencing columns in aForeignKeyConstraint
construct.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”.버전 0.8으로 변경: A multiple-foreign key join ambiguity can be resolved by setting the
foreign_keys
parameter alone, without the need to explicitly setprimaryjoin
as well. - 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
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 theforeign_keys
parameter when presented with an ambiguous condition. In typical cases, ifrelationship()
doesn’t raise any exceptions, theforeign_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.더 보기
Creating Custom Foreign Conditions
foreign()
- allows direct annotation of the “foreign” columns within aprimaryjoin
condition.버전 0.8에 추가: The
foreign()
annotation can also be applied directly to theprimaryjoin
expression, which is an alternate, more specific system of describing which columns in a particularprimaryjoin
should be considered “foreign”. - 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
- info¶ –
Optional data dictionary which will be populated into the
MapperProperty.info
attribute of this object.버전 0.8에 추가.
- 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.더 보기
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.더 보기
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 theinnerjoin
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.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.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. See the section Dynamic Relationship Loaders for more details.- True - a synonym for ‘select’
- False - a synonym for ‘joined’
- None - a synonym for ‘noload’
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Relationship Loading Techniques - Full documentation on relationship loader configuration.
Dynamic Relationship Loaders - detail on the
dynamic
option. - 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
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.더 보기
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 theColumn
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 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.
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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 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.더 보기
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.더 보기
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. - 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.버전 0.8으로 변경: The
remote()
annotation can also be applied directly to theprimaryjoin
expression, which is an alternate, more specific system of describing which columns in a particularprimaryjoin
should be considered “remote”.더 보기
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 asremote_side
, typically when a customprimaryjoin
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 specifieslazy="dynamic"
or was otherwise constructed using theorm.dynamic_loader()
function.더 보기
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. - 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.더 보기
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, setuselist
to False.The
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
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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 specifiesviewonly
can work with a wider range of SQL operations within theprimaryjoin
condition, including operations that feature the use of a variety of comparison operators as well as SQL functions such ascast()
. Theviewonly
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.
- argument¶ –
-
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 torelationship()
in place of a string argument, e.g.:'items':relationship( SomeItem, backref=backref('parent', lazy='subquery'))
-
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 withrelationship()
: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.
버전 0.8에 추가.
-
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.
버전 0.8에 추가.