SQLAlchemy One-to-Many relationship on single table inheritance - declarative

Question

Basically, I have this model, where I mapped in a single table a \"BaseNode\" class, and two subclasses. The point is that I need one of the subclasses, to have a one-to-man

Answer 1

I was struggling through this myself earlier. I was able to get this self-referential relationship working:

class Employee(Base):
  __tablename__ = 'employee'
  id = Column(Integer, primary_key=True)
  name = Column(String(64), nullable=False)
Employee.manager_id = Column(Integer, ForeignKey(Employee.id))
Employee.manager = relationship(Employee, backref='subordinates',
    remote_side=Employee.id)

Note that the manager and manager_id are "monkey-patched" because you cannot make self-references within a class definition.

So in your example, I would guess this:

class NodeTypeA(BaseNode):
    __mapper_args__ = {'polymorphic_identity': 'NodeTypeA'}
    typeB_children = relationship('NodeTypeB', backref='parent_node',
        remote_side='NodeTypeB.parent_id')

EDIT: Basically what your error is telling you is that the relationship and its backref are both identical. So whatever rules that SA is applying to figure out what the table-level relationships are, they don't jive with the information you are providing.

I learned that simply saying mycolumn=relationship(OtherTable) in your declarative class will result in mycolumn being a list, assuming that SA can detect an unambiguous relationship. So if you really want an object to have a link to its parent, rather than its children, you can define parent=relationship(OtherTable, backref='children', remote_side=OtherTable.id) in the child table. That defines both directions of the parent-child relationship.