self-referential many-to-many relationships and mixins

[till.plewe]

I am using python 3.3 and sqlalchemy 0.8.2

I am trying to define a self-referential many-to-many relationship for a
class where the primary key is provided by a mixin. Defining the primary
directly in the class works. Using the mixin does not.

I would be grateful for any suggestions or pointers to relevant documentation.

Below is an example showing my problem.  As given the example works.
Uncommenting the line "#id = ..." in 'Base' and commenting out the
corresponding line in 'A' breaks the example. Is there any way to define
the primary key in Base and getting the 'requires' relation to work?

-----------------------------------

from sqlalchemy import Column, Integer, String, DateTime, Table, ForeignKey,create_engine
from sqlalchemy.ext.declarative import declarative_base,declared_attr
from sqlalchemy.orm import sessionmaker, relationship, backref

class Base(object):
    #id = Column(Integer, primary_key=True)
    pass

Base = declarative_base(cls=Base)

association_table = Table('association',
                          Base.metadata,
                          Column('prerequisite', Integer, ForeignKey('a.id')),
                          Column('dependency', Integer, ForeignKey('a.id')))

class A(Base):
    __tablename__ = "a"
    id = Column(Integer, primary_key=True)
    requires           = relationship("A",
                                      secondary = association_table,
                                      primaryjoin=(id==association_table.c.prerequisite),
                                      secondaryjoin=(id==association_table.c.dependency),
                                      backref = backref("required_by"))

if __name__ == "__main__":
    engine = create_engine('sqlite:///:memory:', echo=False)
    Session = sessionmaker(bind=engine)
    session = Session()
    Base.metadata.create_all(engine)

    T=A()
    U=A()
    session.add(T)
    session.add(U)
    T.requires.append(U)
    session.commit()
    print("T",T.id,T.requires,T.required_by)
    print("U",U.id,U.requires,U.required_by)

[Simon King]

You can make it work by using strings as the primaryjoin and
secondaryjoin parameters and referring to A.id rather than just id:

class A(Base):
__tablename__ = "a"

requires = relationship("A",
secondary = association_table,

primaryjoin="A.id==association.c.prerequisite",

secondaryjoin="A.id==association.c.dependency",

backref = backref("required_by"))

This technique is described in the "Configuring Relationships" section
of the declarative documentation:

http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/declarative.html#configuring-relationships

Hope that helps,

Simon

[till plewe]

Thank you. That does the trick.

Till

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