Many-to-many relationship in the same table as an association object

Question

Related (for the case of using a non-association-object): SQLAlchemy Many-to-many ratio for a single table

Building many-to-many relationships is easy. Creating a many-to-many relationship in the same table is almost as easy as described in the above question.

Creating a many-to-many relationship with an association is also easy.

What I cannot find is the right way to combine the objects of association and many-to-many relationships when the left and right sides are the same table.

So, starting with a simple, naive and obviously wrong version, which I spent forever trying to massage into the correct version:

t_groups = Table('groups', metadata,
    Column('id', Integer, primary_key=True),
)

t_group_groups = Table('group_groups', metadata,
    Column('parent_group_id', Integer, ForeignKey('groups.id'), primary_key=True, nullable=False),
    Column('child_group_id', Integer, ForeignKey('groups.id'), primary_key=True, nullable=False),
    Column('expires', DateTime),
)

mapper(Group_To_Group, t_group_groups, properties={
    'parent_group':relationship(Group),
    'child_group':relationship(Group),
})

?

Answer 1

, , Could not determine join condition between parent/child tables... mapper Group_To_Group :

mapper(Group_To_Group, t_group_groups, properties={
    'parent_group':relationship(Group,
        primaryjoin=(t_group_groups.c.parent_group_id==t_groups.c.id),),
    'child_group':relationship(Group,
        primaryjoin=(t_group_groups.c.child_group_id==t_groups.c.id),),
})

backref, Group.