Hierarchy in forms metaclasses can miss declarated fields in forms

[Manuel Saelices]

This was occurs after [7847]. Ok, [7847] is a good commit for make able to metaclass subclassing.

This is the scenary:

#### model
class FooModel(models.Model):
    name = models.CharField(max_length=100)

#### forms
class GenericForm(forms.Form):
    extra_field = forms.CharField()

class FooForm(GenericForm, forms.ModelForm):
    # to avoid python metaclass conflict error:
    __metaclass__ = type('FooFormMetaclass', (GenericForm.__metaclass__, forms.ModelForm.__metaclass__), {})
    non_existing_field = forms.CharField()

    class Meta:
        model = FooModel

In previous scenario, FooForm should contain both name, extra_field and nonexisting_field. But only appears name field in base_fields.

See this test:

>>> from testapp.forms import FooForm
>>> FooForm.base_fields.keys()
['name']

I will attach a patch thats fixes problem.

[Manuel Saelices]

This patch fix problems

[Manuel Saelices]

It's only a Form naming change in tests

[Manuel Saelices]

Sorry... this is the good patch

[Manuel Saelices]

We put an incorrect ticket link in test... now is "Regression test for #7837" (before was "#6926")

[Manuel Saelices]

Updated patch for [8049]

[Manuel Saelices]

Patch updated for [8335]

[Jacob]

I'm frankly surprised that the business with __metaclass__ works at all -- I don't see why you want to subclass both Form and ModelForm, and I'm not entirely sure it ought to be possible at all.

In fact, you can just make GenericForm a ModelForm and then you don't have this weird metaclass hack.

[Manuel Saelices]

A custom ModelForm class that inherits another custom Form was very useful in many ways. In my company there are several projects where we are using this patch, because is needed that functionality.

For example, you could use a ModelForm with ​WTForm (one of the django snippets most rated), because you want having rapid and flexible layout design in your model forms.

[Manuel Saelices]

I think multiple inneritance of ModelForm and a normal Form shouldn't be problematic. I don't like also use of __metaclass__ = ... but this is only way to avoid python metaclass conflicts.

Could you find other way to achieve this?

[Jacob]

I think this needs to wait for a proper fix -- if it's gonna work, it should work without the __metaclass__ hack. We can make that happen, but it'll be post-1.0.

[Manuel Saelices]

There is a recipe of avoiding metaclass conflicts here: ​http://code.activestate.com/recipes/204197/

[(none)]

Milestone post-1.0 deleted

[Julien Phalip]

Patch needs improvement as per Jacob's comments.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[loic84]

Fixed in #19617.

Note that as documented the ModelForm subclass should appear first in the MRO:

class FooForm(forms.ModelForm, GenericForm):
   pass