Admin enforces default widgets, preventing the use of custom ones

[nanaki]

When you inherit models.DateTimeField (and several other fields) you are unable to change its widget it always uses the default.
The code in django/contrib/admin/options.py (line 68) looks like this:
It forcibly overrides anything you put in widget or form_class.

        if isinstance(db_field, models.DateTimeField):
            kwargs['form_class'] = forms.SplitDateTimeField
            kwargs['widget'] = widgets.AdminSplitDateTime()
            """
            return db_field.formfield(**kwargs)

I tried changing it to this:
But then all my DateTime fields lost their widgets.

        if isinstance(db_field, models.DateTimeField):
            defaults = {'form_class': forms.SplitDateTimeField,
                        'widget': widgets.AdminSplitDateTime()}
            defaults.update(kwargs)
            return db_field.formfield(**kwargs)

This is a simplified example

from django.db import models
from django.forms import Field,DateTimeField
from django.forms.widgets import Widget

from django.utils.safestring import mark_safe

class ReadonlyWidget(Widget):
    def render(self, name, value, attrs=None):
        return mark_safe(u'<span>%s</span>' % value)

class ReadonlyDateTimeField(DateTimeField):
    def formfield(self, **kwargs):
        defaults = {'form_class': Field, 'widget': ReadonlyWidget}
        defaults.update(kwargs)
        return super(ReadonlyDateTimeField, self).formfield(**defaults)

class Ticket(models.Model):
    date_raised = ReadonlyDateTimeField()
    date_closed = models.DateTimeField()

[gregoire]

It is the same problem for every widget specific to the admin.

I wanted to use it with TextField to get a RichTextField with TinyMCE enabled, but I can't. I have to inherit from CharField and reimplement TextField to make it work.

[Jacob]

The current patch on #8306 fixes this also.

[Jacob]

(In [9760]) Cleaned up and refactored ModelAdmin.formfield_for_dbfield:

• The new method uses an admin configuration option (formfield_overrides); this makes custom admin widgets especially easy.
• Refactored what was left of formfield_for_dbfield into a handful of smaller methods so that it's easier to hook in and return custom fields where needed.
• These formfield_for_* methods now pass around request so that you can easily modify fields based on request (as in #3987).

Fixes #8306, #3987, #9148.

Thanks to James Bennet for the original patch; Alex Gaynor and Brian Rosner also contributed.