Queries don't ensure that comparison objects are the correct type

[Randy Barlow]

This is best demonstrated by an example. Suppose you have the following models:

class A(models.Model):
    pass

class B(models.Model):
    pass

class C(models.Model):
    b = models.ForeignKey(B)

Then suppose you did this:

        a = models.A.objects.create()
        b = models.B.objects.create()
        # This works, because the following assertion is True
        self.assertEquals(a.id, b.id)
        c = models.C.objects.create(b=b)

        # Let's try to query for a c where b==b. Works as expected
        self.assertEquals(models.C.objects.get(b=b).b, b)

        # Uh oh, why can I query for b=a? This first one shouldn't pass
        self.assertEquals(models.C.objects.get(b=a).b, b)
        # This one should fail, but it should fail at the .get() by
        # raising an exception, I think
        self.assertEquals(models.C.objects.get(b=a).b, a)

My proposal is that Django should complain when I pass in an instance of A for comparison with an FK to B, since a isn't the same as b. The query seems to just compare their primary keys, and not also ensure that the types are what is expected.

I've created a small django project that demonstrates this problem.

[Randy Barlow]

Django project, demonstrating the problem

[Randy Barlow]

The problem is demonstrated in the unit tests to the app. Thanks for your consideration!

[Randy Barlow]

For clarity, I propose that an exception should be raised by this code:

models.C.objects.get(b=a)

[Eric Holscher]

This seems like something that should either raise an exception, or at least be noted by a warning in the documentation.

[Eric Holscher]

Bah, forgot "Accept ticket" doesn't set its state to Accepted, but makes me the owner :x

[Luke Plant]

The only tricky thing with this is determining what kind of type check to do. If, for example, the a object is actually any kind of proxy to an instance of models.B it should not be rejected (by the normal Pythonic principles of duck typing). So a simple isinstance(the_model_class_we_expect) will not do.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[ANUBHAV JOSHI]

​https://github.com/django/django/pull/2783

[Tim Graham]

ORM changes could use a final review.

[Tim Graham <timograham@…>]

In 34ba86706f0db33d9a0ab44e4abb78703e7262a9:

Fixed #14334 -- Query relation lookups now check object types.

Thanks rpbarlow for the suggestion; and loic, akaariai, and jorgecarleitao
for reviews.

[Florian Apolloner]

This caused at least one issue as described in #23266

[Anssi Kääriäinen <akaariai@…>]

In ae7cb992bca5d211c9456487feb21b84387006eb:

Fixed #23721 -- check_related_objects without calling iter

Refs #14334