I had an answer for pretty much the same question here:
https://groups.google.com/forum/?fromgroups#!searchin/sqlalchemy/friends/sqlalchemy/tOrvG98TLNw/bwFM82-SwXwJ
On Mar 15, 2012, at 5:08 PM, zz elle wrote:
> I transform it into declarative way and its seems to work but it's not
> bidirectional.
> To support bidirectional friendship(A, B friends) i would say that you might
> transform it into 2 unidirectional friendships (A friend of B and B friend of
> A).
> You could perhaps hide this complexity with an association_proxy which
> creator_factory creates both both friendships
>
> class User(Mixin, BASE):
> id = Column(Integer, primary_key=True)
> __tablename__ = 'user'
> friends = None
>
> class FriendShip(Mixin, BASE):
> __tablename__ = 'friendship'
> user_id = Column(Integer, ForeignKey(User.id))
> friend_id = Column(Integer, ForeignKey(User.id))
>
> User.friends = relationship(User, secondary='friendship',
>
> primaryjoin=User.id==FriendShip.user_id,
>
> secondaryjoin=User.id==FriendShip.friend_id)
>
> On Thursday, March 15, 2012 1:03:27 AM UTC+1, Jon Parise wrote:
> I'm considering modeling many-to-many "friend" relationships between
> users using an association table. The tricky aspect is that I'd like
> the association table's entries to be "bidirectional", meaning the
> single entry (A,B) represents friendship in both directions.
> The "standard" way to model this doesn't support that usage:
>
> friendship_table = Table('friendship',
> Column('user_id', Integer, ForeignKey('user.id')),
> Column('friend_id', Integer, ForeignKey('user.id'))
> )
>
> class User(Base):
> id = Column(db.Integer, primary_key=True)
> friends = db.relationship('User',
> secondary=friendship_table,
> primaryjoin=id==friendship_table.c.user_id,
> secondaryjoin=id==friendship_table.c.friend_id)
>
> ... and I understand why it can't work as-is. My question is whether
> or not this type of relationship is possible to represent using
> relationship() (perhaps by using association_proxy()?).
>
> For the time being, I've just been writing per-operation queries in
> User instance methods and @property descriptors, but it would be nice
> to wrap this up in an proper attributed object that plays nicely with
> the session.
>
>
>
>
>
>
> On Thursday, March 15, 2012 1:03:27 AM UTC+1, Jon Parise wrote:
> I'm considering modeling many-to-many "friend" relationships between
> users using an association table. The tricky aspect is that I'd like
> the association table's entries to be "bidirectional", meaning the
> single entry (A,B) represents friendship in both directions.
> The "standard" way to model this doesn't support that usage:
>
> friendship_table = Table('friendship',
> Column('user_id', Integer, ForeignKey('user.id')),
> Column('friend_id', Integer, ForeignKey('user.id'))
> )
>
> class User(Base):
> id = Column(db.Integer, primary_key=True)
> friends = db.relationship('User',
> secondary=friendship_table,
> primaryjoin=id==friendship_table.c.user_id,
> secondaryjoin=id==friendship_table.c.friend_id)
>
> ... and I understand why it can't work as-is. My question is whether
> or not this type of relationship is possible to represent using
> relationship() (perhaps by using association_proxy()?).
>
> For the time being, I've just been writing per-operation queries in
> User instance methods and @property descriptors, but it would be nice
> to wrap this up in an proper attributed object that plays nicely with
> the session.
>
>
>
>
>
>
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