django-filter: resolve field along with lookup expression to pro… (#805)

* django-filter: resolve field along with lookup expression to properly resolve field

* bring back django-filter with method test

* remove dangling comment

* refactor based on better knowledge of django-filters

Author: zbyte64

graphene_django/filter/utils.py

@@ -1,5 +1,6 @@
 import six
 
+from django_filters.utils import get_model_field
 from .filterset import custom_filterset_factory, setup_filterset
 
 
@@ -18,22 +19,12 @@ def get_filtering_args_from_filterset(filterset_class, type):
         if name in filterset_class.declared_filters:
             form_field = filter_field.field
         else:
-            try:
-                field_name, filter_type = name.rsplit("__", 1)
-            except ValueError:
-                field_name = name
-                filter_type = None
-
-            # If the filter type is `isnull` then use the filter provided by
-            # DjangoFilter (a BooleanFilter).
-            # Otherwise try and get a filter based on the actual model field
-            if filter_type != "isnull" and hasattr(model, field_name):
-                model_field = model._meta.get_field(field_name)
-
-                if hasattr(model_field, "formfield"):
-                    form_field = model_field.formfield(
-                        required=filter_field.extra.get("required", False)
-                    )
+            model_field = get_model_field(model, filter_field.field_name)
+            filter_type = filter_field.lookup_expr
+            if filter_type != "isnull" and hasattr(model_field, "formfield"):
+                form_field = model_field.formfield(
+                    required=filter_field.extra.get("required", False)
+                )
 
         # Fallback to field defined on filter if we can't get it from the
         # model field