id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
24974	Cannot create subclass of form created by `modelform_factory()`, e.g. in `ModelAdmin.get_form()`	Tai Lee	Yoong Kang Lim	"The default implementation of `ModelAdmin.get_form()` uses `modelform_factory()` to dynamically create a new `ModelForm` class, and apply form field overrides, etc.

The docs mention that I can specify a new base form that is passed to this implementation or just return a new `ModelForm` class. But when designing a generic `ModelAdmin` class that will be used with a number of yet unspecified models, I want to use the form returned by the super class as the base, in case one has already been set on a `ModelAdmin` class my class will be mixed into.

I would normally expect that this just works:

{{{
def get_form(self, *args, **kwargs):
    form_class = super(MyModelAdmin, self).get_form(*args, **kwargs)
    class Form(form_class):
        # do stuff.
    return Form
}}}

But while `form_class` does all the things like form field overrides, `Form` does not. All form fields are rendered in the admin with regular form widgets, not the admin variants. The reason appears to be because when `Form` is created, `ModelFormMetaclass` is executed again but without the `formfield_overrides` argument.

To make it work again, I need to do:

{{{
def get_form(self, *args, **kwargs):
    form_class = super(MyModelAdmin, self).get_form(*args, **kwargs)
    class Form(form_class):
        # do stuff.
    kwargs['form'] = Form
    return super(MyModelAdmin, self).get_form(*args, **kwargs)
}}}

This runs through the whole default implementation of `get_form()` twice, but it does work.

Alternatively, if I wanted to override the `__init__()` method on the form, I could replace it instead of creating a subclass, with something like:

{{{
def get_form(self, *args, **kwargs):
    form_class = super(FluentLayoutsMixin, self).get_form(*args, **kwargs)
    old_init = form_class.__init__
    def new_init(self, *args, **kwargs):
        old_init.__get__(self, type(self))(*args, **kwargs)
        # do stuff.
    form_class.__init__ = new_init.__get__(None, form_class)
    return form_class
}}}

This won't execute the default `get_form()` implementation twice, but it's getting pretty hairy.

This took three people a few hours to work out, stepping through many `pdb` sessions in django admin and model form code. A note in the docs might save someone else the trouble.

Something like:

    Note: You cannot just create and return a subclass of form from the super class in this method. Due to the way `ModelFormMetaclass` works, the subclass will ignore `formfield_overrides` unless you pass it back through the default implementation.

{{{
# NO: `formfield_overrides` not applied.
def get_form(self, *args, **kwargs):
    form_class = super(MyModelAdmin, self).get_form(*args, **kwargs)
    class Form(form_class):
        # do stuff.
    return Form

# YES: `formfield_overrides` applied.
def get_form(self, *args, **kwargs):
    form_class = super(MyModelAdmin, self).get_form(*args, **kwargs)
    class Form(form_class):
        # do stuff.
    kwargs['form'] = Form
    return super(MyModelAdmin, self).get_form(*args, **kwargs)
}}}

This is actually more of a general problem with `modelform_factory()` than a problem with `ModelAdmin.get_form()`, so perhaps a note for the `modelform_factory()` docs instead, with a link to it from the `ModelAdmin.get_form()` docs.

Here is an interactive shell session demonstrating the problem with `modelform_factory()`.

{{{
>>> from django import forms
>>> from django.forms.models import modelform_factory
>>> from django.contrib.sites.models import Site

# Create a form class where all fields are converted to `DateTimeField` via callback.
>>> Form = modelform_factory(Site, formfield_callback=lambda f: forms.DateTimeField)
>>> Form.base_fields
OrderedDict([(u'id', <class 'django.forms.fields.DateTimeField'>), ('domain', <class 'django.forms.fields.DateTimeField'>), ('name', <class 'django.forms.fields.DateTimeField'>)])

# Note that for any subclass, all base fields have reverted to their original values, instead of being inherited.
>>> class FooForm(Form): pass
>>> FooForm.base_fields
OrderedDict([('domain', <django.forms.fields.CharField object at 0x112db5f90>), ('name', <django.forms.fields.CharField object at 0x112d74a90>)])
}}}"	Bug	closed	Forms	1.8	Normal	fixed	form modelform modelform_factory modeladmin get_site metaclass modelformmetaclass		Accepted	1	0	0	0	0	0