Allow overriding of ModelChoiceField.choices

[Simon Litchfield]

This patch fixes a validation bug when attempting to override choices manually on a ModelChoiceField. Comments in code suggest this should be able to be done.

Note, this used to work until the recent model/form validation changes.

[Simon Litchfield]

Note the patch has also swept up the addition of 'BaseModelFormSet', 'BaseInlineFormSet' to all (they can be useful!)

[Greg Turner]

-1 on the addition of BaseModelFormSet to __all__. Although the classes can be useful, they are abstract, and can be imported explicitly with:

from forms.models import BaseModelFormSet

For the problem at hand, a test that illustrates the bug would help immensely. Reviewing the patch, it looks reasonable, except the choice of variable name for pk, given that something else could be in self.to_field_name.

[Matthias Kestenholz]

The patch needs documentation, tests and reasons why BaseModelFormSet and BaseInlineFormset should be added to all.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tommy Beadle]

I can't think of a use case where it makes sense to add abitrary choices to the ones available for a ModelChoiceField via its queryset. See ​http://stackoverflow.com/questions/5281195/forms-modelchoicefield-queryset-extra-choice-fields-django-forms for a way to use a normal ChoiceField to accomplish this.

If the desire is to change the appearance of the choices when they are rendered, there is the ModelChoiceField.label_from_instance method that can be overridden.