python - Dependency rule tried to blank out primary key in SQLAlchemy, when foreign key constraint is part of composite primary key

Question

I have the following model definitions

class Foo(Base):
    __tablename__ = 'foo'

    id = Column(Integer, primary_key=True)
    name = Column(String(200))


class FooCycle(Base):
    __tablename__ = 'foocycle'

    foo_id = Column(
        String(50),
        ForeignKey('foo.id'),
        primary_key=True
    )
    some_number = Column(
        Integer,
        primary_key=True,
    )

    foo = relationship("Foo", backref="cycles")

and the following test case

class HierarchicModelTest(unittest.TestCase):
    def test_create_data_via_orm_save_twice(self):
        # get_session is a convenience wrapper to access a scoped session object
        s = get_session()

        def create_foo():
            foo = Foo(id="12345", name="fancy foo")
            foo.cycles = [FooCycle(some_number=1)]

            return foo

        # initially create foo
        foo = create_foo()
        s.add(foo)
        s.flush()

        # recreating foo, using merge to update into database
        foo = create_foo()
        s.merge(foo)

        # raises Exception: Dependency rule tried to blank-out primary key
        # column 'foocycle.foo_id' on instance '<FooCycle at 0x32e6b10>'
        s.flush()

The test fails with a neat little stack trace and the final assertion error, telling me that the "Dependency rule tried to blank-out primary key column 'foocycle.foo_id". I'm assuming SQLAlchemy cannot, or doesn't want to calculate the value for foo_id on FooCycle itself. I can explicitly set this value myself in create_foo:

def create_foo():
    foo = Foo(id="12345", name="fancy foo")
    foo.cycles = [FooCycle(some_number=1, foo_id="12345")]

    return foo

But, due to conciseness, architectural considerations and admittedly personal pride I don't want to. Is there a simple way to get SQLAlchemy to resolve this issue. I haven't quite grasped the purpose of the dependency rule. Any pointers/information on that issue?

Stack Trace:

# Test 1 of 7:
# test_core.HierarchicModelTest.test_create_data_via_orm_save_twice
===============
HierarchicModelTest: test_create_data_via_orm_save_twice (tests.test_core.HierarchicModelTest)
Failed test "test_create_data_via_orm_save_twice (tests.test_core.HierarchicModelTest)"! Reason: Dependency rule tried to blank-out primary key column 'foocycle.foo_id' on instance '<FooCycle at 0x39cda10>'
Traceback (most recent call last):
  File "/home/xxx/xxx/xxx/backend/tests/test_core.py", line 115, in test_create_data_via_orm_save_twice
    s.flush()
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 149, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1879, in flush
    self._flush(objects)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1997, in _flush
    transaction.rollback(_capture_exception=True)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 57, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1961, in _flush
    flush_context.execute()
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 370, in execute
    rec.execute(self)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 479, in execute
    self.dependency_processor.process_saves(uow, states)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/dependency.py", line 552, in process_saves
    uowcommit, False)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/dependency.py", line 569, in _synchronize
    sync.clear(dest, self.mapper, self.prop.synchronize_pairs)
  File "/home/xxx/xxx/xxx/.env/lib/python2.7/site-packages/sqlalchemy/orm/sync.py", line 53, in clear
    (r, orm_util.state_str(dest))
AssertionError: Dependency rule tried to blank-out primary key column 'foocycle.foo_id' on instance '<FooCycle at 0x39cda10>'

Answer 1

Based on the comment by van I was able to work out a solution. The default relationship cascade is "save-update, merge". I had to set this to "save-update, merge, delete, delete-orphan".

Adding delete by itself did not change the behavior, delete-orphan was necessary.

Adding only delete-orphan made a deletion testcase fail, with the "dependency rule" assertion error, mentioned in the question:

class HierarchicModelTest(unittest.TestCase):
    def test_delete_parent_object(self):
        foo = Foo(**foo_data).save()
        self.assertEqual(Foo.query.count(), 1)
        self.assertEqual(FooCycle.query.count(), 1)

        s = get_session()
        s.delete(foo)
        s.flush()

        self.assertEqual(Foo.query.count(), 0)
        self.assertEqual(FooCycle.query.count(), 0)

--

  File "/home/xxx/xxx/xxx/backend/tests/test_core.py", line 128, in test_delete_parent_object
     s.flush()
  [...]
  AssertionError: Dependency rule tried to blank-out primary key column 'foocycle.foo_id' on instance '<FooCycle at 0x37a1710>'

From the SQLAlchemy docs:

delete-orphan cascade adds behavior to the delete cascade, such that a child object will be marked for deletion when it is de-associated from the parent, not just when the parent is marked for deletion.

So, the correct definition of the FooCycle Model is

class FooCycle(Base):
    __tablename__ = 'foocycle'

    foo_id = Column(
        String(50),
        ForeignKey('foo.id'),
        primary_key=True
    )
    some_number = Column(
        Integer,
        primary_key=True,
    )

    foo = relationship("Foo",
                       backref=backref("cycles",
                                        cascade="save-update, merge, "
                                                "delete, delete-orphan"))