[Sqlalchemy-tickets] Issue #4308: Cascade behavior with uselist=False and association_proxy (zzzeek/sqlalchemy)

[Dmytro S. <iss...@bi...>]

New issue 4308: Cascade behavior with uselist=False and association_proxy
https://bitbucket.org/zzzeek/sqlalchemy/issues/4308/cascade-behavior-with-uselist-false-and

Dmytro Starosud:

This is using sqlalchemy 1.2.10.

Please consider following code snippet

```
#!python
    Base = declarative_base()

    class HasID:
        id = Column(Integer, primary_key=True)

    class A(Base, HasID):
        __tablename__ = 'test_a'
        ab = relationship('AB', backref='a', cascade='all, delete-orphan', uselist=False)
        b = association_proxy('ab', 'b', creator=lambda b: AB(b=b))

    class B(Base, HasID):
        __tablename__ = 'test_b'
        ab = relationship('AB', backref='b', cascade='all, delete-orphan')

    class AB(Base, HasID):
        __tablename__ = 'test_ab'
        a_id = Column(Integer, ForeignKey(A.id), nullable=False)
        b_id = Column(Integer, ForeignKey(B.id), nullable=False)

    Base.metadata.create_all(session.bind)

    a = A(b=B())
    session.add(a)
    session.commit()

    a.b = None
    session.commit()  # throws
    # > cursor.execute(statement, parameters)
    # E sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) null value in column "b_id" violates not-null constraint
    # E DETAIL:  Failing row contains (1, 1, null).
    # E  [SQL: 'UPDATE test_ab SET b_id=%(b_id)s WHERE test_ab.id = %(test_ab_id)s'] [parameters: {'test_ab_id': 1, 'b_id': None}]
```

Is this just an unfortunate coincidence or I misused API?
Thanks a lot in advance!