Remove unused function, raise ValueError if prefetch_source is wrong value

Author: Maxime Toussaint

README.md

@@ -196,7 +196,7 @@ class SomeSerializer(PrefetchingSerializerMixin, serializer.ModelSerializer):
         )
 ```
 
-**IMPORTANT NOTE**: Because of an issue with Django, behaviour could be inconsistant when using to_attr with Prefetch objects. For more information, see: https://code.djangoproject.com/ticket/34791
+**IMPORTANT NOTE**: Because of an issue with Django, behaviour could be inconsistant when using to_attr with Prefetch objects. For more information, see: <https://code.djangoproject.com/ticket/34791>
 
 ----
 

serializer_prefetch/base.py

@@ -254,7 +254,14 @@ def _get_serializer_field_relations(
             if not is_prefetch_object and not is_model_field(model, source):
                 if getattr(field, "_prefetch_source", None):
                     raise ValueError(
-                        _('The field "{}" is not a model field.').format(source)
+                        _(
+                            'The prefetch_source "{}" is not a valid value for '
+                            "field {} on serializer {}."
+                        ).format(
+                            source,
+                            field.source,
+                            serializer.__class__.__name__,
+                        )
                     )
 
                 continue

serializer_prefetch/utils.py

@@ -64,13 +64,6 @@ def get_custom_related(related_attr, current_relation=None):
     return computed_related
 
 
-def transform_str_to_prefetch(item):
-    if isinstance(item, str):
-        return Prefetch(item)
-
-    return item
-
-
 def get_model_from_serializer(serializer):
     with suppress(AttributeError):
         return serializer.Meta.model

tests/test_conditions.py

@@ -375,3 +375,19 @@ class Meta:
 
         with self.assertNumQueries(1):
             serializer.data
+
+        # Passing a wrong prefetch_source should raise a ValueError
+        class PizzaSerializer(PrefetchingSerializerMixin, serializers.ModelSerializer):
+            provenance = CountrySerializer(
+                source="provenance_", prefetch_source="wrong_source"
+            )
+
+            class Meta:
+                model = Pizza
+                fields = ("label", "provenance")
+
+        pizzas = list(Pizza.objects.all())
+        serializer = PizzaSerializer(pizzas, many=True)
+
+        with self.assertRaises(ValueError):
+            serializer.data