On Tue, Aug 13, 2013 at 2:07 PM, till.plewe <[email protected]> wrote:
> 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)
>
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
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