Improved error when apps registry is not populated (#1204)

django_filters/utils.py

@@ -164,10 +164,18 @@ def get_field_parts(model, field_name):
             return None
 
         fields.append(field)
-        if isinstance(field, RelatedField):
-            opts = field.remote_field.model._meta
-        elif isinstance(field, ForeignObjectRel):
-            opts = field.related_model._meta
+        try:
+            if isinstance(field, RelatedField):
+                opts = field.remote_field.model._meta
+            elif isinstance(field, ForeignObjectRel):
+                opts = field.related_model._meta
+        except AttributeError:
+            # Lazy relationships are not resolved until registry is populated.
+            raise RuntimeError(
+                "Unable to resolve relationship `%s` for `%s`. Django is most "
+                "likely not initialized, and its apps registry not populated. "
+                "Ensure Django has finished setup before loading `FilterSet`s."
+                % (field_name, model._meta.label))
 
     return fields
 

tests/test_utils.py

@@ -222,6 +222,22 @@ def test_reverse_related_field(self):
         self.assertIsInstance(parts[1], models.ManyToManyField)
         self.assertIsInstance(parts[2], models.CharField)
 
+    def test_lazy_relationship_not_ready(self):
+        """
+        This simulates trying to create a FilterSet before the app registry has
+        been populated. Lazy relationships have not yet been resolved from their
+        strings into their remote model referencess.
+        """
+        class TestModel(models.Model):
+            fk = models.ForeignKey('remote.Model', on_delete=models.CASCADE)
+
+        msg = ("Unable to resolve relationship `fk__f` for `tests.TestModel`. "
+               "Django is most likely not initialized, and its apps registry "
+               "not populated. Ensure Django has finished setup before loading "
+               "`FilterSet`s.")
+        with self.assertRaisesMessage(RuntimeError, msg):
+            get_field_parts(TestModel, 'fk__f')
+
 
 class GetModelFieldTests(TestCase):