associationproxy
is used to create a read/write view of a
target attribute across a relationship. It essentially conceals
the usage of a “middle” attribute between two endpoints, and
can be used to cherry-pick fields from a collection of
related objects or to reduce the verbosity of using the association
object pattern. Applied creatively, the association proxy allows
the construction of sophisticated collections and dictionary
views of virtually any geometry, persisted to the database using
standard, transparently configured relational patterns.
Consider a many-to-many mapping between two classes, User
and Keyword
.
Each User
can have any number of Keyword
objects, and vice-versa
(the many-to-many pattern is described at Many To Many):
from sqlalchemy import Column, Integer, String, ForeignKey, Table
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
kw = relationship("Keyword", secondary=lambda: userkeywords_table)
def __init__(self, name):
self.name = name
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
userkeywords_table = Table('userkeywords', Base.metadata,
Column('user_id', Integer, ForeignKey("user.id"),
primary_key=True),
Column('keyword_id', Integer, ForeignKey("keyword.id"),
primary_key=True)
)
Reading and manipulating the collection of “keyword” strings associated
with User
requires traversal from each collection element to the .keyword
attribute, which can be awkward:
>>> user = User('jek')
>>> user.kw.append(Keyword('cheese inspector'))
>>> print(user.kw)
[<__main__.Keyword object at 0x12bf830>]
>>> print(user.kw[0].keyword)
cheese inspector
>>> print([keyword.keyword for keyword in user.kw])
['cheese inspector']
The association_proxy
is applied to the User
class to produce
a “view” of the kw
relationship, which only exposes the string
value of .keyword
associated with each Keyword
object:
from sqlalchemy.ext.associationproxy import association_proxy
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
kw = relationship("Keyword", secondary=lambda: userkeywords_table)
def __init__(self, name):
self.name = name
# proxy the 'keyword' attribute from the 'kw' relationship
keywords = association_proxy('kw', 'keyword')
We can now reference the .keywords
collection as a listing of strings,
which is both readable and writable. New Keyword
objects are created
for us transparently:
>>> user = User('jek')
>>> user.keywords.append('cheese inspector')
>>> user.keywords
['cheese inspector']
>>> user.keywords.append('snack ninja')
>>> user.kw
[<__main__.Keyword object at 0x12cdd30>, <__main__.Keyword object at 0x12cde30>]
The AssociationProxy
object produced by the association_proxy()
function
is an instance of a Python descriptor.
It is always declared with the user-defined class being mapped, regardless of
whether Declarative or classical mappings via the mapper()
function are used.
The proxy functions by operating upon the underlying mapped attribute or collection in response to operations, and changes made via the proxy are immediately apparent in the mapped attribute, as well as vice versa. The underlying attribute remains fully accessible.
When first accessed, the association proxy performs introspection operations on the target collection so that its behavior corresponds correctly. Details such as if the locally proxied attribute is a collection (as is typical) or a scalar reference, as well as if the collection acts like a set, list, or dictionary is taken into account, so that the proxy should act just like the underlying collection or attribute does.
When a list append() event (or set add(), dictionary __setitem__(), or scalar assignment event) is intercepted by the association proxy, it instantiates a new instance of the “intermediary” object using its constructor, passing as a single argument the given value. In our example above, an operation like:
user.keywords.append('cheese inspector')
Is translated by the association proxy into the operation:
user.kw.append(Keyword('cheese inspector'))
The example works here because we have designed the constructor for Keyword
to accept a single positional argument, keyword
. For those cases where a
single-argument constructor isn’t feasible, the association proxy’s creational
behavior can be customized using the creator
argument, which references a
callable (i.e. Python function) that will produce a new object instance given the
singular argument. Below we illustrate this using a lambda as is typical:
class User(Base):
# ...
# use Keyword(keyword=kw) on append() events
keywords = association_proxy('kw', 'keyword',
creator=lambda kw: Keyword(keyword=kw))
The creator
function accepts a single argument in the case of a list-
or set- based collection, or a scalar attribute. In the case of a dictionary-based
collection, it accepts two arguments, “key” and “value”. An example
of this is below in Proxying to Dictionary Based Collections.
The “association object” pattern is an extended form of a many-to-many relationship, and is described at Association Object. Association proxies are useful for keeping “association objects” out the way during regular use.
Suppose our userkeywords
table above had additional columns
which we’d like to map explicitly, but in most cases we don’t
require direct access to these attributes. Below, we illustrate
a new mapping which introduces the UserKeyword
class, which
is mapped to the userkeywords
table illustrated earlier.
This class adds an additional column special_key
, a value which
we occasionally want to access, but not in the usual case. We
create an association proxy on the User
class called
keywords
, which will bridge the gap from the user_keywords
collection of User
to the .keyword
attribute present on each
UserKeyword
:
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, backref
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
# association proxy of "user_keywords" collection
# to "keyword" attribute
keywords = association_proxy('user_keywords', 'keyword')
def __init__(self, name):
self.name = name
class UserKeyword(Base):
__tablename__ = 'user_keyword'
user_id = Column(Integer, ForeignKey('user.id'), primary_key=True)
keyword_id = Column(Integer, ForeignKey('keyword.id'), primary_key=True)
special_key = Column(String(50))
# bidirectional attribute/collection of "user"/"user_keywords"
user = relationship(User,
backref=backref("user_keywords",
cascade="all, delete-orphan")
)
# reference to the "Keyword" object
keyword = relationship("Keyword")
def __init__(self, keyword=None, user=None, special_key=None):
self.user = user
self.keyword = keyword
self.special_key = special_key
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
def __repr__(self):
return 'Keyword(%s)' % repr(self.keyword)
With the above configuration, we can operate upon the .keywords
collection of each User
object, and the usage of UserKeyword
is concealed:
>>> user = User('log')
>>> for kw in (Keyword('new_from_blammo'), Keyword('its_big')):
... user.keywords.append(kw)
...
>>> print(user.keywords)
[Keyword('new_from_blammo'), Keyword('its_big')]
Where above, each .keywords.append()
operation is equivalent to:
>>> user.user_keywords.append(UserKeyword(Keyword('its_heavy')))
The UserKeyword
association object has two attributes here which are populated;
the .keyword
attribute is populated directly as a result of passing
the Keyword
object as the first argument. The .user
argument is then
assigned as the UserKeyword
object is appended to the User.user_keywords
collection, where the bidirectional relationship configured between User.user_keywords
and UserKeyword.user
results in a population of the UserKeyword.user
attribute.
The special_key
argument above is left at its default value of None
.
For those cases where we do want special_key
to have a value, we
create the UserKeyword
object explicitly. Below we assign all three
attributes, where the assignment of .user
has the effect of the UserKeyword
being appended to the User.user_keywords
collection:
>>> UserKeyword(Keyword('its_wood'), user, special_key='my special key')
The association proxy returns to us a collection of Keyword
objects represented
by all these operations:
>>> user.keywords
[Keyword('new_from_blammo'), Keyword('its_big'), Keyword('its_heavy'), Keyword('its_wood')]
The association proxy can proxy to dictionary based collections as well. SQLAlchemy
mappings usually use the attribute_mapped_collection()
collection type to
create dictionary collections, as well as the extended techniques described in
Custom Dictionary-Based Collections.
The association proxy adjusts its behavior when it detects the usage of a
dictionary-based collection. When new values are added to the dictionary, the
association proxy instantiates the intermediary object by passing two
arguments to the creation function instead of one, the key and the value. As
always, this creation function defaults to the constructor of the intermediary
class, and can be customized using the creator
argument.
Below, we modify our UserKeyword
example such that the User.user_keywords
collection will now be mapped using a dictionary, where the UserKeyword.special_key
argument will be used as the key for the dictionary. We then apply a creator
argument to the User.keywords
proxy so that these values are assigned appropriately
when new elements are added to the dictionary:
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, backref
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm.collections import attribute_mapped_collection
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
# proxy to 'user_keywords', instantiating UserKeyword
# assigning the new key to 'special_key', values to
# 'keyword'.
keywords = association_proxy('user_keywords', 'keyword',
creator=lambda k, v:
UserKeyword(special_key=k, keyword=v)
)
def __init__(self, name):
self.name = name
class UserKeyword(Base):
__tablename__ = 'user_keyword'
user_id = Column(Integer, ForeignKey('user.id'), primary_key=True)
keyword_id = Column(Integer, ForeignKey('keyword.id'), primary_key=True)
special_key = Column(String)
# bidirectional user/user_keywords relationships, mapping
# user_keywords with a dictionary against "special_key" as key.
user = relationship(User, backref=backref(
"user_keywords",
collection_class=attribute_mapped_collection("special_key"),
cascade="all, delete-orphan"
)
)
keyword = relationship("Keyword")
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
def __repr__(self):
return 'Keyword(%s)' % repr(self.keyword)
We illustrate the .keywords
collection as a dictionary, mapping the
UserKeyword.string_key
value to Keyword
objects:
>>> user = User('log')
>>> user.keywords['sk1'] = Keyword('kw1')
>>> user.keywords['sk2'] = Keyword('kw2')
>>> print(user.keywords)
{'sk1': Keyword('kw1'), 'sk2': Keyword('kw2')}
Given our previous examples of proxying from relationship to scalar
attribute, proxying across an association object, and proxying dictionaries,
we can combine all three techniques together to give User
a keywords
dictionary that deals strictly with the string value
of special_key
mapped to the string keyword
. Both the UserKeyword
and Keyword
classes are entirely concealed. This is achieved by building
an association proxy on User
that refers to an association proxy
present on UserKeyword
:
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship, backref
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm.collections import attribute_mapped_collection
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
# the same 'user_keywords'->'keyword' proxy as in
# the basic dictionary example
keywords = association_proxy(
'user_keywords',
'keyword',
creator=lambda k, v:
UserKeyword(special_key=k, keyword=v)
)
def __init__(self, name):
self.name = name
class UserKeyword(Base):
__tablename__ = 'user_keyword'
user_id = Column(Integer, ForeignKey('user.id'), primary_key=True)
keyword_id = Column(Integer, ForeignKey('keyword.id'),
primary_key=True)
special_key = Column(String)
user = relationship(User, backref=backref(
"user_keywords",
collection_class=attribute_mapped_collection("special_key"),
cascade="all, delete-orphan"
)
)
# the relationship to Keyword is now called
# 'kw'
kw = relationship("Keyword")
# 'keyword' is changed to be a proxy to the
# 'keyword' attribute of 'Keyword'
keyword = association_proxy('kw', 'keyword')
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
User.keywords
is now a dictionary of string to string, where
UserKeyword
and Keyword
objects are created and removed for us
transparently using the association proxy. In the example below, we illustrate
usage of the assignment operator, also appropriately handled by the
association proxy, to apply a dictionary value to the collection at once:
>>> user = User('log')
>>> user.keywords = {
... 'sk1':'kw1',
... 'sk2':'kw2'
... }
>>> print(user.keywords)
{'sk1': 'kw1', 'sk2': 'kw2'}
>>> user.keywords['sk3'] = 'kw3'
>>> del user.keywords['sk2']
>>> print(user.keywords)
{'sk1': 'kw1', 'sk3': 'kw3'}
>>> # illustrate un-proxied usage
... print(user.user_keywords['sk3'].kw)
<__main__.Keyword object at 0x12ceb90>
One caveat with our example above is that because Keyword
objects are created
for each dictionary set operation, the example fails to maintain uniqueness for
the Keyword
objects on their string name, which is a typical requirement for
a tagging scenario such as this one. For this use case the recipe
UniqueObject, or
a comparable creational strategy, is
recommended, which will apply a “lookup first, then create” strategy to the constructor
of the Keyword
class, so that an already existing Keyword
is returned if the
given name is already present.
The AssociationProxy
features simple SQL construction capabilities
which relate down to the underlying relationship()
in use as well
as the target attribute. For example, the RelationshipProperty.Comparator.any()
and RelationshipProperty.Comparator.has()
operations are available, and will produce
a “nested” EXISTS clause, such as in our basic association object example:
>>> print(session.query(User).filter(User.keywords.any(keyword='jek')))
SELECT user.id AS user_id, user.name AS user_name
FROM user
WHERE EXISTS (SELECT 1
FROM user_keyword
WHERE user.id = user_keyword.user_id AND (EXISTS (SELECT 1
FROM keyword
WHERE keyword.id = user_keyword.keyword_id AND keyword.keyword = :keyword_1)))
For a proxy to a scalar attribute, __eq__()
is supported:
>>> print(session.query(UserKeyword).filter(UserKeyword.keyword == 'jek'))
SELECT user_keyword.*
FROM user_keyword
WHERE EXISTS (SELECT 1
FROM keyword
WHERE keyword.id = user_keyword.keyword_id AND keyword.keyword = :keyword_1)
and .contains()
is available for a proxy to a scalar collection:
>>> print(session.query(User).filter(User.keywords.contains('jek')))
SELECT user.*
FROM user
WHERE EXISTS (SELECT 1
FROM userkeywords, keyword
WHERE user.id = userkeywords.user_id
AND keyword.id = userkeywords.keyword_id
AND keyword.keyword = :keyword_1)
AssociationProxy
can be used with Query.join()
somewhat manually
using the attr
attribute in a star-args context:
q = session.query(User).join(*User.keywords.attr)
New in version 0.7.3: attr
attribute in a star-args context.
attr
is composed of AssociationProxy.local_attr
and AssociationProxy.remote_attr
,
which are just synonyms for the actual proxied attributes, and can also
be used for querying:
uka = aliased(UserKeyword)
ka = aliased(Keyword)
q = session.query(User).\
join(uka, User.keywords.local_attr).\
join(ka, User.keywords.remote_attr)
New in version 0.7.3: AssociationProxy.local_attr
and AssociationProxy.remote_attr
,
synonyms for the actual proxied attributes, and usable for querying.
sqlalchemy.ext.associationproxy.
association_proxy
(target_collection, attr, **kw)¶Return a Python property implementing a view of a target attribute which references an attribute on members of the target.
The returned value is an instance of AssociationProxy
.
Implements a Python property representing a relationship as a collection of simpler values, or a scalar value. The proxied property will mimic the collection type of the target (list, dict or set), or, in the case of a one to one relationship, a simple scalar value.
Parameters: |
|
---|
sqlalchemy.ext.associationproxy.
AssociationProxy
(target_collection, attr, creator=None, getset_factory=None, proxy_factory=None, proxy_bulk_set=None, info=None)¶Bases: sqlalchemy.orm.base.InspectionAttrInfo
A descriptor that presents a read/write view of an object attribute.
__init__
(target_collection, attr, creator=None, getset_factory=None, proxy_factory=None, proxy_bulk_set=None, info=None)¶Construct a new AssociationProxy
.
The association_proxy()
function is provided as the usual
entrypoint here, though AssociationProxy
can be instantiated
and/or subclassed directly.
Parameters: |
|
---|
any
(criterion=None, **kwargs)¶Produce a proxied ‘any’ expression using EXISTS.
This expression will be a composed product
using the RelationshipProperty.Comparator.any()
and/or RelationshipProperty.Comparator.has()
operators of the underlying proxied attributes.
attr
¶Return a tuple of (local_attr, remote_attr)
.
This attribute is convenient when specifying a join
using Query.join()
across two relationships:
sess.query(Parent).join(*Parent.proxied.attr)
New in version 0.7.3.
See also:
contains
(obj)¶Produce a proxied ‘contains’ expression using EXISTS.
This expression will be a composed product
using the RelationshipProperty.Comparator.any()
, RelationshipProperty.Comparator.has()
,
and/or RelationshipProperty.Comparator.contains()
operators of the underlying proxied attributes.
extension_type
= symbol('ASSOCIATION_PROXY')¶has
(criterion=None, **kwargs)¶Produce a proxied ‘has’ expression using EXISTS.
This expression will be a composed product
using the RelationshipProperty.Comparator.any()
and/or RelationshipProperty.Comparator.has()
operators of the underlying proxied attributes.
info
¶info
attribute of InspectionAttrInfo
Info dictionary associated with the object, allowing user-defined
data to be associated with this InspectionAttr
.
The dictionary is generated when first accessed. Alternatively,
it can be specified as a constructor argument to the
column_property()
, relationship()
, or composite()
functions.
New in version 0.8: Added support for .info to all
MapperProperty
subclasses.
Changed in version 1.0.0: MapperProperty.info
is also
available on extension types via the
InspectionAttrInfo.info
attribute, so that it can apply
to a wider variety of ORM and extension constructs.
is_aliased_class
= False¶is_attribute
= False¶is_clause_element
= False¶is_instance
= False¶is_mapper
= False¶is_property
= False¶is_selectable
= False¶local_attr
¶The ‘local’ MapperProperty
referenced by this
AssociationProxy
.
New in version 0.7.3.
See also:
remote_attr
¶The ‘remote’ MapperProperty
referenced by this
AssociationProxy
.
New in version 0.7.3.
See also:
scalar
¶Return True
if this AssociationProxy
proxies a scalar
relationship on the local side.
target_class
¶The intermediary class handled by this AssociationProxy
.
Intercepted append/set/assignment events will result in the generation of new instances of this class.
sqlalchemy.ext.associationproxy.
ASSOCIATION_PROXY
= symbol('ASSOCIATION_PROXY')¶