SQLAlchemy multiple foreign keys on the same column

Question

I am working with the following database table design. The relationship is one-to-many, parent to child. So one parent (mother or father) has multiple children. I'm having issues mapping these tables because the child's column parent_id is a Foreign Key to both of the parent tables.

class Mother(Base):
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.unique_id'))


class Father(Base):
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.unique_id'))


Class Children(Base):
    id = Column(Integer, primary_key=True)
    unique_id = Column(Integer, ForeignKey('mother.child_id')

I have tried defining a relationship with backref in the parent tables to avoid explicitely defining Foreign Keys in the child tables:

class Mother(Base):    
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.unique_id'))
    children = relationship('Child', backref='mother')


class Father(Base):
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.unique_id'))
    children = relationship('Child', backref='father')


Class Children(Base):
    id = Column(Integer, primary_key=True)
    unique_id = Column(Integer)

But this yields a one-to-one relationship.

Answer 1

I think that foreign keys should bo on Child table, it would be better and more realistic model:

class Mother(Base):    
    id = Column(Integer, primary_key=True)
    children = relationship('Child', backref='mother')

class Father(Base):
    id = Column(Integer, primary_key=True)
    children = relationship('Child', backref='father')

class Child(Base):
    id = Column(Integer, primary_key=True)
    father_id = Column(Integer, ForeignKey('father.id'))
    mother_id = Column(Integer, ForeignKey('mother.id'))

In above model, each child has at most one mother and one father, while a mother/father can have many children.