foreign key validate() violates router API when no instance specified

[Ben Demboski]

to reproduce:

1. configure two databases: default and other
2. have two models Person and Pet:

   class Person(models.Model):
       name = models.CharField(max_length=100)
   
   class Pet(models.Model):
       name = models.CharField(max_length=100)
       owner = models.ForeignKey(Person)
   

3. have a database router enabled that routes models in the app containing person and pet (let's say its called petstore) to the other database:

   class PetStoreRouter(object):
       def db_for_read(self, model, **hints):
           if model._meta.app_label == 'petstore':
               return 'other'
           return None
   
       def db_for_write(self, model, **hints):
           if model._meta.app_label == 'petstore':
               return 'other'
           return None
   

4. create a Person instance named person
5. call clean() on the Pet model's owner field passing in persons pk, but no model:

   Pet._meta.get_field('owner').clean(person.pk, None)
   

The result will be a exception thrown from inside the router's db_for_read() method:

AttributeError: type object 'NoneType' has no attribute '_meta'

because the model argument contains <type 'NoneType'> (which doesn't have a _meta attribute) instead of a model class.

[Ben Demboski]

Patch is:
​https://github.com/django/django/pull/6813

[Tim Graham <timograham@…>]

In 2224a566:

Fixed #26784 -- Made ForeignKey.validate() pass model to router if model_instance=None.