Thanks Mike for the quick reply.

I have changed the code so it uses backref on one side. The models now get 
created w/o error. I haven't actually tested using the relationship but I'm 
sure it's fine now.

For anyone else with this problem, here's the 'fixed' code. I didn't change 
the Artifact_Reference table, it's the same as posted above. I'm not keen 
on the naming master/slave.. maybe primary/secondary would be better.. It's 
not exactly Parent/Child.. heh.


class Artifact_Relation(Base):
    __tablename__ = 'artifact_relation'
    master_artifact_id = Column(
        Integer,
        ForeignKey('artifact.id', name='artifact_relation_master_id_fk', 
ondelete="cascade", onupdate="cascade"),
        primary_key=True,
        nullable=False
    )

    slave_artifact_id = Column(
        Integer,
        ForeignKey('artifact.id', name='artifact_relation_slave_id_fk', 
ondelete="cascade", onupdate="cascade"),
        primary_key=True,
        nullable=False
    )

    relationship_type = Column(String(24), nullable=False)

    slave = relationship("Artifact", backref=backref("masters"), 
foreign_keys=[slave_artifact_id])
    master = relationship("Artifact", backref=backref("slaves"), 
foreign_keys=[master_artifact_id])

class Artifact(Base):
    "Artifact"
    __tablename__ = 'artifact'
    id = Column('id', Integer, primary_key=True)
    artifact_type = Column('artifact_type', String(16), nullable=False)
    __mapper_args__ = {
        'polymorphic_on': artifact_type
    }

    artifact_references = relationship(
        'Artifact_Reference',
        back_populates='artifact',
        cascade="all, delete-orphan",
        passive_deletes=True
    )



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