- argument –
a mapped class, or actual Mapper instance, representing the
target of the relationship.
:paramref:`~.relationship.argument` may also be passed as a callable function
which is evaluated at mapper initialization time, and may be passed as a
Python-evaluable string when using Declarative.
See also
declarative_configuring_relationships - further detail
on relationship configuration when using Declarative.
- secondary –
for a many-to-many relationship, specifies the intermediary
table, and is typically an instance of Table.
In less common circumstances, the argument may also be specified
as an Alias construct, or even a Join construct.
:paramref:`~.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 Table that is
present in the MetaData collection associated with the
parent-mapped Table.
The :paramref:`~.relationship.secondary` keyword argument is typically
applied in the case where the intermediary Table is not
otherwise exprssed in any direct class mapping. If the “secondary” table
is also explicitly mapped elsewhere
(e.g. as in association_pattern), one should consider applying
the :paramref:`~.relationship.viewonly` flag so that this relationship()
is not used for persistence operations which may conflict with those
of the association object pattern.
See also
relationships_many_to_many - Reference example of “many to many”.
orm_tutorial_many_to_many - ORM tutorial introduction to
many-to-many relationships.
self_referential_many_to_many - Specifics on using many-to-many
in a self-referential case.
declarative_many_to_many - Additional options when using
Declarative.
association_pattern - an alternative to :paramref:`~.relationship.secondary`
when composing association table relationships, allowing additional
attributes to be specified on the association table.
composite_secondary_join - 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 of
attributes.get_history() which also need to know
the “previous” value of the attribute.
- backref –
indicates the string name of a property to be placed on the related
mapper’s class that will handle this relationship in the other
direction. The other property will be created automatically
when the mappers are configured. Can also be passed as a
backref() object to control the configuration of the
new relationship.
- back_populates –
Takes a string name and has the same meaning as :paramref:`~.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:`~.relationship.back_populates`
to this relationship to ensure proper functioning.
- 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. The default value is "save-update, merge".
Available cascades are:
- save-update - cascade the Session.add()
operation. This cascade applies both to future and
past calls to Session.add(),
meaning new items added to a collection or scalar relationship
get placed into the same session as that of the parent, and
also applies to items which have been removed from this
relationship but are still part of unflushed history.
- merge - cascade the Session.merge()
operation
- expunge - cascade the Session.expunge()
operation
- delete - cascade the Session.delete()
operation
- delete-orphan - if an item of the child’s type is
detached from its parent, mark it for deletion.
Changed in version 0.7: This option does not prevent
a new instance of the child object from being persisted
without a parent to start with; to constrain against
that case, ensure the child’s foreign key column(s)
is configured as NOT NULL
- refresh-expire - cascade the Session.expire()
and Session.refresh() operations
- all - shorthand for “save-update,merge, refresh-expire,
expunge, delete”
See also
unitofwork_cascades - Introductory documentation and
examples.
tutorial_delete_cascade - Tutorial example describing
a delete cascade.
- cascade_backrefs=True –
a boolean value indicating if the save-update cascade should
operate along an assignment event intercepted by a backref.
When set to False,
the attribute managed by this relationship will not cascade
an incoming transient object into the session of a
persistent parent, if the event is received via backref.
That is:
mapper(A, a_table, properties={
'bs':relationship(B, backref="a", cascade_backrefs=False)
})
If an A() is present in the session, assigning it to
the “a” attribute on a transient B() will not place
the B() into the session. To set the flag in the other
direction, i.e. so that A().bs.append(B()) won’t add
a transient A() into the session for a persistent B():
mapper(A, a_table, properties={
'bs':relationship(B,
backref=backref("a", cascade_backrefs=False)
)
})
See also
backref_cascade - Introductory documentation and
examples.
- 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
custom_collections - Introductory documentation and
examples.
- comparator_factory –
a class which extends RelationshipProperty.Comparator which
provides custom SQL clause generation for comparison operations.
See also
PropComparator - some detail on redefining comparators
at this level.
custom_comparators - 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.
See also
loading_toplevel - includes an introduction to subquery
eager loading.
- doc – docstring which will be applied to the resulting descriptor.
- extension –
an AttributeExtension instance, or list of extensions,
which will be prepended to the list of attribute listeners for
the resulting descriptor placed on the class.
Deprecated since version 0.7: 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’s :paramref:`~.relationship.primaryjoin` condition.
That is, if the :paramref:`~.relationship.primaryjoin`
condition of this relationship() is a.id ==
b.a_id, and the values in b.a_id are required to be
present in a.id, then the “foreign key” column of this
relationship() is b.a_id.
In normal cases, the :paramref:`~.relationship.foreign_keys`
parameter is not required. relationship() will
automatically determine which columns in the
:paramref:`~.relationship.primaryjoin` conditition are to be
considered “foreign key” columns based on those
Column objects that specify ForeignKey,
or are otherwise listed as referencing columns in a
ForeignKeyConstraint construct.
:paramref:`~.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 the
relationship() to consider just those columns specified
here as “foreign”.
- The Table being mapped does not actually have
ForeignKey or ForeignKeyConstraint
constructs present, often because the table
was reflected from a database that does not support foreign key
reflection (MySQL MyISAM).
- The :paramref:`~.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 the
:paramref:`~.relationship.foreign_keys` parameter when
presented with an ambiguous condition. In typical cases,
if relationship() doesn’t raise any exceptions, the
:paramref:`~.relationship.foreign_keys` parameter is usually
not needed.
:paramref:`~.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.
- 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.
See also
what_kind_of_loading - 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
self_referential_eager_loading - Introductory documentation
and examples.
- lazy=’select’ –
specifies
how the related items should be loaded. Default value is
select. Values include:
- select - items should be loaded lazily when the property is first
accessed, using a separate SELECT statement, or identity map
fetch for simple many-to-one references.
- immediate - items should be loaded as the parents are loaded,
using a separate SELECT statement, or identity map fetch for
simple many-to-one references.
- joined - items should be loaded “eagerly” in the same query as
that of the parent, using a JOIN or LEFT OUTER JOIN. Whether
the join is “outer” or not is determined by the innerjoin
parameter.
- subquery - items should be loaded “eagerly” as the parents are
loaded, using one additional SQL statement, which issues a JOIN to a
subquery of the original statement, for each collection requested.
- 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-configured
Query object for all read
operations, onto which further filtering operations can be
applied before iterating the results. See
the section dynamic_relationship for more details.
- True - a synonym for ‘select’
- False - a synonym for ‘joined’
- None - a synonym for ‘noload’
See also
/orm/loading - Full documentation on relationship loader
configuration.
dynamic_relationship - 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 a Session but is not part of its pending
collection.
The :paramref:`~.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. :paramref:`~.relationship.order_by` is expected to refer to one
of the Column
objects to which the target class is mapped, or
the attribute itself bound to the target class which refers
to the column.
:paramref:`~.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.
- 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.
See also
passive_deletes - Introductory documentation
and examples.
- passive_updates=True –
Indicates loading and INSERT/UPDATE/DELETE behavior when the
source of a foreign key value changes (i.e. an “on update”
cascade), which are typically the primary key columns of the
source row.
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. Note that with databases which enforce
referential integrity (i.e. PostgreSQL, MySQL with InnoDB tables),
ON UPDATE CASCADE is required for this operation. The
relationship() will update the value of the attribute on related
items which are locally present in the session during a flush.
When False, it is assumed that the database does not enforce
referential integrity and will not be issuing its own CASCADE
operation for an update. The relationship() will issue the
appropriate UPDATE statements to the database in response to the
change of a referenced key, and items locally present in the
session during a flush will also be refreshed.
This flag should probably be set to False if primary key changes
are expected and the database in use doesn’t support CASCADE
(i.e. SQLite, MySQL MyISAM tables).
See also
passive_updates - Introductory documentation and
examples.
:paramref:`.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 :paramref:`~.relationship.post_update` to “break” the cycle.
See also
post_update - 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).
:paramref:`~.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.
See also
relationship_primaryjoin
- remote_side –
used for self-referential relationships, indicates the column or
list of columns that form the “remote side” of the relationship.
:paramref:`.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.
Changed in version 0.8: The remote() annotation can also be applied
directly to the primaryjoin expression, which is an alternate,
more specific system of describing which columns in a particular
primaryjoin should be considered “remote”.
- query_class –
a Query subclass that will be used as the base of the
“appender query” returned by a “dynamic” relationship, that
is, a relationship that specifies lazy="dynamic" or was
otherwise constructed using the orm.dynamic_loader()
function.
See also
dynamic_relationship - 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.
:paramref:`~.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.
See also
relationship_primaryjoin
- single_parent –
when True, installs a validator which will prevent objects
from being associated with more than one parent at a time.
This is used for many-to-one or many-to-many relationships that
should be treated either as one-to-one or one-to-many. Its usage
is optional, except for relationship() constructs which
are many-to-one or many-to-many and also
specify the delete-orphan cascade option. The relationship()
construct itself will raise an error instructing when this option
is required.
- uselist –
a boolean that indicates if this property should be loaded as a
list or a scalar. In most cases, this value is determined
automatically by relationship() at mapper configuration
time, based on the type and direction
of the relationship - one to many forms a list, many to one
forms a scalar, many to many is a list. If a scalar is desired
where normally a list would be present, such as a bi-directional
one-to-one relationship, set :paramref:`~.relationship.uselist` to False.
The :paramref:`~.relationship.uselist` flag is also available on an
existing relationship() construct as a read-only attribute, which
can be used to determine if this relationship() deals with
collections or scalar attributes:
>>> User.addresses.property.uselist
True
- viewonly=False – when set to True, the relationship is used only for loading objects,
and not for any persistence operation. A relationship()
which specifies :paramref:`~.relationship.viewonly` can work
with a wider range of SQL operations within the :paramref:`~.relationship.primaryjoin`
condition, including operations that feature the use of
a variety of comparison operators as well as SQL functions such
as cast(). The :paramref:`~.relationship.viewonly`
flag is also of general use when defining any kind of relationship()
that doesn’t represent the full set of related objects, to prevent
modifications of the collection from resulting in persistence operations.