Custom field on admin.py - FieldDoesNotExist when using RelatedFieldWidgetWrapper

[Naran Khetani]

Hello Users,

Hoping for someone to help me with this, I have a admin form which I am overwriting as below:

admin.py

def add_related_field_wrapper(form, col_name):
    rel_model = form.Meta.model
    rel = rel_model._meta.get_field(col_name).rel
    form.fields[col_name].widget = RelatedFieldWidgetWrapper(
        form.fields[col_name].widget,
        rel,
        admin.site,
        can_add_related=True,
        can_change_related=True
    )


class DocumentItemsForm(forms.ModelForm):

    tags = ModelMultipleChoiceField(label="Tags", required=False, queryset=Terms.objects.filter(termtypeid__name='tag'))
    categorys = ModelMultipleChoiceField(label="Categorys", required=False, queryset=Terms.objects.filter(termtypeid__name='category'))

    def __init__(self, *args, **kwargs):
        super(DocumentItemsForm, self).__init__(*args, **kwargs)
        add_related_field_wrapper(self, 'tags')

@admin.register(Documentitems)
class DocumentitemsAdmin(admin.ModelAdmin):

    form = DocumentItemsForm
    save_as = True

I get the below exception:

Django Version: 2.1.7
Exception Type: FieldDoesNotExist
Exception Value: Documentitems has no field named 'tags'

The actual model does not have those fields above, I have a feeling its looking up the actual model which is why its not finding the field. In essence what im trying to do is add a custom field that does not exist in the model which works fine, however i need to be able to add the plus button next to the field which allows the user to add a new record and ive be lead to the above solution to achieve this.