virtual_field.py

"""
ModelForm mixin that supports virtual fields, ie. GenericForeignKey.
By default, Django's ModelForm will not update the instance's virtual fields
even if a matching form field was given to the form. For example::
    class YourForm(forms.ModelForm):
        gfk_name = SomeField()
    instance = YourModel.objects.get(pk=4)
    # Django doesn't populate virtual fields by default, compensate manually:
    initial = {'gfk_name': instance.gfk_name}
    # Use the form as usual
    form = YourForm(request.POST, instance=instance, initial=initial)
    instance = form.save()
    # instance.gfk_name will not be set, compensate manually:
    instance.gfk_ctype = ContentType.objects.get_for_model(
        instance.cleaned_data['gfk_name'])
    instance.gfk_object_id = instance.cleaned_data['gfk_name'].pk
With YourForm(GenericForeignKeyModelFormMixin, forms.ModelForm) instead of
YourForm(forms.ModelForm), GenericForeignKeys are supported automatically.
"""


from django.contrib.contenttypes.models import ContentType


class GenericForeignKeyModelFormMixin(object):
    """
    Enable virtual field (generic foreign key) handling in django's ModelForm.
    - treat virtual fields like GenericForeignKey as normal fields,
    - when setting a GenericForeignKey value, also set the object id and
      content type id fields.
    Probably, django doesn't do that for legacy reasons: virtual fields were
    added after ModelForm and simply nobody asked django to add virtual field
    support in ModelForm.
    """
    def __init__(self, *args, **kwargs):
        """
        The constructor adds virtual field values to
        :py:attr:`django:django.forms.Form.initial`
        """
        super(GenericForeignKeyModelFormMixin, self).__init__(*args, **kwargs)

        # do what model_to_dict doesn't
        for field in self._meta.model._meta.virtual_fields:
            try:
                self.initial[field.name] = getattr(
                    self.instance,
                    field.name,
                    None
                )
            except:
                continue

    def _post_clean(self):
        """
        What ModelForm does, but also set virtual field values from
        cleaned_data.
        """
        super(GenericForeignKeyModelFormMixin, self)._post_clean()

        # take care of virtual fields since django doesn't
        for field in self._meta.model._meta.virtual_fields:
            value = self.cleaned_data.get(field.name, None)

            if value:
                setattr(self.instance, field.name, value)

                self.cleaned_data[field.ct_field] = \
                    ContentType.objects.get_for_model(value)
                self.cleaned_data[field.fk_field] = value.pk