sqlalchemy updates another model before deletion

Question

I'm using sqlalchemy with postgresql. And I'm newbie of sqlalchemy.

I made forien key for model "User" called "to_user_id" to model "Invitation" and this key is not nullable.

When I try to delete instance of model "User" using

session.delete(user)

And sqlalchemy set invitation's to_user_id to NULL automatically before deletion and postgresql raise following error.

IntegrityError: (IntegrityError) null value in column "to_user_id" violates not-null constraint

How can I disable it?

Here's my model's definition

class User(Base):
    '''
    User model
    '''
    __tablename__='User'
    id = Column(Integer,primary_key=True)

class Invitation(Base):
    '''
    Invitation model
    '''
    __tablename__ = 'Invitation'
    __table_args__ = (UniqueConstraint('appointment_id', 'to_user_id'),)
    id = Column(Integer, primary_key=True)

    appointment_id = Column(Integer,ForeignKey('Appointment.id',
        ondelete='CASCADE'), nullable=False)
    appointment = relationship('Appointment', backref=backref('invitations'),
    )

    from_user_id = Column(Integer,ForeignKey('User.id',
        ondelete='SET NULL'), nullable=True)
    from_user = relationship('User', backref=backref('sent_invitations'),
        primaryjoin='Invitation.from_user_id==User.id')

    to_user_id = Column(Integer,ForeignKey('User.id',
        ondelete='CASCADE'), nullable=False)
    to_user = relationship('User',backref=backref('received_invitations'),
        primaryjoin='Invitation.to_user_id==User.id',
    )

Answer 1

You should pass cascade='delete' to the to_user relationship(). See the docs here.

The ondelete argument to ForeignKey affects the generation of DDL statements, not how the ORM behaves.