GenericForeignKey crashes if content_type_id is changed and object_id is type incompatible with old object

[Richard Laager]

Steps to reproduce:

1. Create a model ("A") with a GenericForeignKey.
2. Create an instance of A ("a") referencing an instance of model "B" with a PK that is an integer type.
3. Change the instance of "A" to reference an instance of model "C" with a PK that is incompatible with int(). But make this change using content_type_id and object_id properties, not content_object, i.e.:

       a.content_type_id = ContentType.objects.get_for_model(B)
       a.object_id = "foo"
   

4. Try to access a.content_object.

Expected result:

This looks up and returns an instance of "b" (the B with a PK of "foo").

Actual result:

This crashes (I'm using 3.2, but the code is unchanged in master):

  File "django/db/models/fields/__init__.py", line 1836, in to_python
    return int(value)

  ValueError: invalid literal for int() with base 10: 'foo'

This happens because it tries to to_python() the new key on the old model's PK field.

One possible fix would be to make the logic short-circuit, like this (also attached):

django/contrib/contenttypes/fields.py

@@ -242 +242 @@
             ct_match = (
                 ct_id == self.get_content_type(obj=rel_obj, using=instance._state.db).id
             )
-            pk_match = rel_obj._meta.pk.to_python(pk_val) == rel_obj.pk
-            if ct_match and pk_match:
-                return rel_obj
-            else:
-                rel_obj = None
+            if ct_match:
+                pk_match = rel_obj._meta.pk.to_python(pk_val) == rel_obj.pk
+                if pk_match:
+                    return rel_obj
+            rel_obj = None
         if ct_id is not None:
             ct = self.get_content_type(id=ct_id, using=instance._state.db)
             try: