How do I create and save dynamic fields in Django

I have some models:

class GroupType(models.Model):
    name = models.CharField(max_length=255)

class Group(models.Model):
    name = models.CharField(max_length=255)
    group_type = models.ForeignKey(GroupType)

class Person(models.Model):
    name = models.CharField(max_length=255)
    groups = models.ManyToManyField(Group, related_name="+")

But in the ModelAdmin I'd like to dynamically add fields for each group type. So if I have two group types public, private, I'd like the admin form to show two fields public_groups and private_groups instead of the actual db field groups.

More Info

I've tried creating a custom form to add the fields dynamically:

class PersonAdminForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(PersonAdminForm, self).__init__(*args, **kwargs)
        new_fields = {}
        for group_type in GroupType.objects.all():
            field_name = "{0}_groups".format(group_type.name.lower())
            qs = Group.objects.filter(group_type=group_type)
            field_field = forms.ModelMultipleChoiceField(queryset=qs)
            new_fields[field_name] = field_field
        self.fields.update(new_fields)

    class Meta:
        model = Person
        fields = '__all__'

Which seems to do the trick as far as adding the fields to the form. But saving those fields and and adding them to the PersonAdmin doesn't work. If I add the fields explicitly to the fields attribute on the PersonAdmin I get:

FieldError: Unknown field(s) (public_groups, private_groups) specified for Person. Check fields/fieldsets/exclude attributes of class PersonAdmin.

I also get the same thing when trying to add them "dynamically" through a custom get_formsets method:

def get_fieldsets(self, request, obj=None):
    fieldsets = super(PersonAdmin, self).get_fieldsets(request, obj)
    print(self.form.fields)
    fieldsets[0][1]['fields'].extend(['public_groups', 'private_groups'])
    return fieldsets

The following code worked perfectly. The overridden __init__ and clean methods on the ModelForm adds the dynamic fields and defines how the values should be saved.

The overridden get_form and get_fieldsets together with the fieldsets attribute on the AdminModel make sure the dynamic form fields get displayed in the admin.

class PersonAdminForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(PersonAdminForm, self).__init__(*args, **kwargs)
        new_fields = {}
        initial = self.instance.groups.all()
        for group_type in GroupType.objects.all():
            field_name = '{0}_groups'.format(group_type.name.lower())
            qs = Group.objects.filter(group_type=group_type)
            field = forms.ModelMultipleChoiceField(
                queryset=qs,
                required=False,
                initial=initial,
            )
            new_fields[field_name] = field
        self.fields.update(new_fields)

    def clean(self):
        cleaned_data = super(PersonAdminForm, self).clean()
        groups = []
        for group_type in GroupType.objects.all():
            gt_name = '{0}_groups'.format(group_type.name.lower())
            groups.extend(cleaned_data.get(gt_name))
        self.instance.groups.clear()
        self.instance.groups.add(*groups)
        return cleaned_data

    class Meta:
        model = Person
        fields = '__all__'


@admin.register(Person)
class PersonAdmin(admin.ModelAdmin):
    form = PersonAdminForm

    # using the fieldsets attribute instead of fields in order to dynamically
    # add group type fields later.
    fieldsets = (
        (None, {
            'fields': (
                'name',
            ),
        }),
    )

    def get_form(self, request, obj=None, **kwargs):
        kwargs['fields'] = flatten_fieldsets(self.declared_fieldsets)
        return super(PersonAdmin, self).get_form(request, obj, **kwargs)

    def get_fieldsets(self, request, obj=None):
        fieldsets = super(PersonAdmin, self).get_fieldsets(request, obj)
        newfieldsets = list(fieldsets)
        fields = []
        for group_type in GroupType.objects.all():
            fields.append('{0}_groups'.format(group_type.name.lower()))
        newfieldsets.append(['Groups', {'fields': fields}])
        return newfieldsets