Set-based association proxy through AppenderQuery?

[Mark Friedenbach]

Hi,

Is it possible to have an association_proxy (in the association object
pattern) that emulates a set-based collection if it goes through a
lazy='dynamic' relationship? I can't for the life of me find a way to
make this work (setting collection_class on the dynamic relationship
doesn't seem to do anything).

Here's some example code of what I'm trying to do, extracted from the
actual project:

class ProofOfWork(object):
blocks = association_proxy('Intermediatory_nodes', 'block')
proof_of_work = Table('proof_of_work', db.metadata)
mapper(ProofOfWork, proof_of_work, properties={
'Intermediatory_nodes': relationship(lambda: Intermediatory,
lazy = 'dynamic'),
})

class Block(object):
proof_of_works = association_proxy('Intermediatory_nodes',
'proof_of_work')
block = Table('block', db.metadata)
mapper(Block, block, properties={
'Intermediatory_nodes': relationship(lambda: Intermediatory,
lazy = 'dynamic'),
})

class Intermediatory(object):
pass
intermediatory = Table('intermediatory', db.metadata,
Column('proof_of_work_id', Integer,
ForeignKey('proof_of_work.id'),
nullable = False),
Column('block_id', Integer,
ForeignKey('block.id')),
)
mapper(Intermediatory, intermediatory, properties={
'proof_of_work': relationship(lambda: ProofOfWork,
back_populates = 'Intermediatory_nodes',
remote_side = lambda: proof_of_work.c.id),
'block': relationship(lambda: Block,
back_populates = 'Intermediatory_nodes',
remote_side = lambda: block.c.id),
})

How can I make ProofOfWork.blocks and Block.proof_of_works return an
_AssociationSet instead of _AssociationList?

Cheers,
Mark

[Jon Parise]

I have a similar use case, and aside from introducing a duplicate "non-lazy"

relationship to back the association_proxy, I haven't found a solution.

Does anyone have a more elegant approach?

[Michael Bayer]

association proxy documents the "proxy_factory" attribute for this purpose.  see below.

from sqlalchemy import *

from sqlalchemy.orm import *

from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.ext.associationproxy import association_proxy, _AssociationSet

import operator

Base = declarative_base()

class AppenderAssociationSet(_AssociationSet):

    """subclass _AssociationSet to adapt some set methods to that of

    AppenderQuery.

    """

    def add(self, object_):

        self.col.append(self._create(object_))

    def extend(self, objects):

        for obj in objects:

            self.col.append(self._create(obj))

    def clear(self):

        """"the set assignment needs 'clear' but we dont

        really have a consistent way to do that with

        AppenderQuery. """

def set_factory(lazy_collection, creator, value_attr, assoc_prox):

    """Factory for associationproxy collections."""

    # does "return MyObject.<value_attr>"

    getter = operator.attrgetter(value_attr)

    # does "MyObject.<value_attr> = <v>"

    setter = lambda o, v: setattr(o, value_attr, v)

    return AppenderAssociationSet(lazy_collection, creator,

                    getter, setter, assoc_prox)

class A(Base):

    __tablename__ = "a"

    id = Column(Integer, primary_key=True)

    bs = relationship("B", lazy="dynamic")

    cs = association_proxy("bs", "c", proxy_factory=set_factory)

class B(Base):

    __tablename__ = "b"

    id = Column(Integer, primary_key=True)

    def __init__(self, c):

        self.c = c

    a_id = Column(Integer, ForeignKey('a.id'))

    c_id = Column(Integer, ForeignKey('c.id'))

    c = relationship("C")

class C(Base):

    __tablename__ = "c"

    id = Column(Integer, primary_key=True)

e = create_engine("sqlite://", echo=True)

Base.metadata.create_all(e)

c1, c2, c3 = C(), C(), C()

s = Session(e)

s.add_all([

        A(cs=set([c1, c2]))

])

s.commit()

a1 = s.query(A).first()

print a1.cs

a1.cs.add(c3)

s.commit()

print a1.cs.difference([c1])

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