refactor(backend/mixins): make AggregateQuerysetMixin more readable

backend/timed/mixins.py

@@ -3,6 +3,16 @@
 from timed.serializers import AggregateObject
 
 
+def is_related_field(field: relations.Field) -> bool:
+    """Check whether value is a related field.
+
+    Ignores serializer method fields which define logic separately.
+    """
+    return isinstance(field, relations.ResourceRelatedField) and not isinstance(
+        field, relations.ManySerializerMethodResourceRelatedField
+    )
+
+
 class AggregateQuerysetMixin:
     """Add support for aggregate queryset in view.
 
@@ -18,59 +28,43 @@ class AggregateQuerysetMixin:
     Mixin expects the id to be the same key as the resource related
     field defined in the serializer.
 
-    To reduce number of queries `prefetch_related_for_field` can be defined
-    to prefetch related data per field like the following:
     >>> from rest_framework.viewsets import ReadOnlyModelViewSet
     ...
     ...
     ... class MyView(ReadOnlyModelViewSet, AggregateQuerysetMixin):
     ...     # ...
-    ...     prefetch_related_for_field = {"field_name": ["field_name_prefetch"]}
-    ...     # ...
     """
 
-    def _is_related_field(self, val):
-        """Check whether value is a related field.
-
-        Ignores serializer method fields which define logic separately.
-        """
-        return isinstance(val, relations.ResourceRelatedField) and not isinstance(
-            val, relations.ManySerializerMethodResourceRelatedField
-        )
-
-    def get_serializer(self, data=None, *args, **kwargs):
+    def _get_data(self, data, *args, **kwargs):
         # no data no wrapping needed
         if not data:
-            return super().get_serializer(data, *args, **kwargs)
+            return data
 
         many = kwargs.get("many")
-        if not many:
-            data = [data]
 
         # prefetch data for all related fields
+
         prefetch_per_field = {}
         serializer_class = self.get_serializer_class()
-        for key, value in serializer_class._declared_fields.items():  # noqa: SLF001
-            if self._is_related_field(value):
-                source = value.source or key
-                if many:
-                    obj_ids = data.values_list(source, flat=True)
-                else:
-                    obj_ids = [data[0][source]]
-
-                qs = value.model.objects.filter(id__in=obj_ids)
-                qs = qs.select_related()
-                if hasattr(self, "prefetch_related_for_field"):  # pragma: no cover
-                    qs = qs.prefetch_related(
-                        *self.prefetch_related_for_field.get(source, [])
-                    )
-
-                objects = {obj.id: obj for obj in qs}
-                prefetch_per_field[source] = objects
+
+        lookup_expr = "id__in" if many else "id"
+
+        for key, value in filter(
+            lambda kv: is_related_field(kv[1]),
+            serializer_class._declared_fields.items(),  # noqa: SLF001
+        ):
+            source = value.source or key
+
+            lookup_value = data.values_list(source, flat=True) if many else data[source]
+
+            qs = value.model.objects.filter(**{lookup_expr: lookup_value})
+            qs = qs.select_related()
+
+            prefetch_per_field[source] = {obj.id: obj for obj in qs}
 
         # enhance entry dicts with model instances
-        data = [
-            AggregateObject(
+        def _construct_aggregate_object(entry):
+            return AggregateObject(
                 **{
                     **entry,
                     **{
@@ -79,10 +73,13 @@ def get_serializer(self, data=None, *args, **kwargs):
                     },
                 }
             )
-            for entry in data
-        ]
 
         if not many:
-            data = data[0]
+            return _construct_aggregate_object(data)
+
+        return [_construct_aggregate_object(entry) for entry in data]
 
-        return super().get_serializer(data, *args, **kwargs)
+    def get_serializer(self, data=None, *args, **kwargs):
+        return super().get_serializer(
+            self._get_data(data, *args, **kwargs), *args, **kwargs
+        )