Disabled ModelChoiceFields crash in Django 1.10

[karyon]

i have several tests that fail when trying to submit a form that has disabled fields in 1.10, which worked in 1.9, and also works when making it disabled=False. example stacktrace:

Traceback (most recent call last):
  File "/vagrant/evap/staff/tests/test_forms.py", line 105, in test_single_result_form_saves_participant_and_voter_count
    self.assertTrue(form.is_valid())
  File "/vagrant/src/django/django/forms/forms.py", line 161, in is_valid
    return self.is_bound and not self.errors
  File "/vagrant/src/django/django/forms/forms.py", line 153, in errors
    self.full_clean()
  File "/vagrant/src/django/django/forms/forms.py", line 364, in full_clean
    self._post_clean()
  File "/vagrant/src/django/django/forms/models.py", line 401, in _post_clean
    self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
  File "/vagrant/src/django/django/forms/models.py", line 60, in construct_instance
    f.save_form_data(instance, cleaned_data[f.name])
  File "/vagrant/src/django/django/db/models/fields/__init__.py", line 867, in save_form_data
    setattr(instance, self.name, data)
  File "/vagrant/src/django/django/db/models/fields/related_descriptors.py", line 203, in __set__
    self.field.remote_field.model._meta.object_name,
ValueError: Cannot assign "1": "Course.semester" must be a "Semester" instance.

code for the form is ​here (it's about the semester attribute, and ignore the two methods, this also fails when removing those) and test is ​here.

this regressed in db19619545dd99a1d2502c72974d79eca33acff7.

[Ryan Schave]

Added unit test to duplicate problem

[Tim Graham]

Claude, I'm not sure how to proceed here. It seems that the assumption about "Initial values are supposed to be clean" isn't correct for foreign keys. They're converted to primary key in model_to_dict and (previously) back to model instances in clean(). Now that latter conversion is skipped.

[Claude Paroz]

If we don't find a better solution, we could partially revert that faulty part of the patch, like I did when backporting to 1.9 (#25532). But then a disabled JSONField will be non-functional. It may be less critical, though.

[Tim Graham]

​PR to partially revert and add the regression test from Ryan.

I created #26949 for the issue about disabled JSONField's crashing.

[Tim Graham <timograham@…>]

In a5f85d89:

Fixed #26917 -- Fixed crash in disabled ModelChoiceFields.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

[Tim Graham <timograham@…>]

In 3744fc1:

[1.10.x] Fixed #26917 -- Fixed crash in disabled ModelChoiceFields.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

Backport of a5f85d891b51d7ceb4f9e422e3e4f5c741062288 from master