EasyFieldsets mixin for Django admin

from django.utils.datastructures import SortedDict
from django.db.models.fields import AutoField


class EasyFieldsets(object):
    """
    A mixin for use with ``ModelAdmin`` and related inline classes allowing
    fieldsets to be used without having to manually define all fields
    (undefined ones are placed at the end of the ``None`` fieldset).
    """

    def __init__(self, *args, **kwargs):
        super(EasyFieldsets, self).__init__(*args, **kwargs)
        fieldsets = SortedDict(self.fieldsets)
        # Get the currently named fields from the fieldsets.
        named_fields = []
        for fieldset in fieldsets.values():
            for field in fieldset['fields']:
                if isinstance(field, basestring):
                    named_fields.append(field)
                else:
                    # Fields can be grouped.
                    named_fields.extend(field)
        # Add any missing fields into the main fieldset.
        fields = self.model._meta.fields + self.model._meta.many_to_many
        missing_fields = [f.name for f in fields
                          if f.editable and not isinstance(f, AutoField)
                          and f.name not in named_fields]
        if missing_fields:
            if None not in fieldsets:
                fieldsets[None] = {}
                # Push the missing base fieldset to the top of the fieldset
                # order.
                fieldsets.keyOrder.remove(None)
                fieldsets.keyOrder.insert(0, None)
            fields = tuple(missing_fields)
            base_fieldset = fieldsets.get(None)
            base_fieldset['fields'] = (tuple(base_fieldset.get('fields', []))
                                       + tuple(missing_fields))
        self.fieldsets = fieldsets.items()