[BUGFIX] Fixed EagerLoadingMixin early return error

api_v2/views/mixins/eager_loading_mixin.py

@@ -14,50 +14,38 @@ class (ie. ReadOnlyModelViewSet).
 
   ## Usage Example
   ```
-    # EagerLoadingMixin inhertired before base-case
     class CreatureViewSet(EagerLoadingMixin, viewsets.ReadOnlyModelViewSet):
       queryset = models.Creature.objects.all().order_by('pk')
       serializer_class = serializers.CreatureSerializer
       filterset_class = CreatureFilterSet
-
       select_related_fields = []   # ForeignKey relations to optimise with select_related()      
       prefetch_related_fields = [] # ManyToMany/reverse relations to optimise with prefetch_related()
   ```
   """
 
   # Override these lists in child views 
-  select_related_fields = [] # ForeignKeys to optimise
-  prefetch_related_fields = [] # ManyToMany & reverse relationships to prefetch
+  select_related_fields = []
+  prefetch_related_fields = []
 
   def get_queryset(self):
-    """Override DRF's default get_queryset() method to apply eager loading"""
     queryset = super().get_queryset()
-
-    # Get query parameters from request
     requested_fields = self.request.query_params.get('fields', '').split(',')
-    depth = int(self.request.query_params.get('depth', 0))
+    filtered_select_fields = self.filter_fields(self.select_related_fields, requested_fields)
+    filtered_prefetch_fields = self.filter_fields(self.prefetch_related_fields, requested_fields)
 
-    # if no fields are passed via query param, select/prefetch all fields defined on the view
-    if not requested_fields:
-      queryset = queryset.select_related(*self.select_related_fields)
-      queryset = queryset.prefetch_related(*self.prefetch_related_fields)
-      return queryset
-
-    # filter selected fields against fields requested by user via query params 
-    # this stops Django prefetching data that isn't even returned by this view
-    select_fields = []
-    for field_to_select in self.select_related_fields:
-      if any(field_in_request in field_to_select for field_in_request in requested_fields):
-        select_fields.append(field_to_select)
-
-    # filter prefetch fields against fields requested by user via query params
-    # this stops Django prefetching data that isn't even returned by this view
-    prefetch_fields = []
-    for field_to_prefetch in self.prefetch_related_fields:
-      if any(field_in_request in field_to_prefetch for field_in_request in requested_fields):
-        prefetch_fields.append(field_to_prefetch)
+    return queryset \
+      .select_related(*filtered_select_fields) \
+      .prefetch_related(*filtered_prefetch_fields)
 
-    # Apply filtered optimisations
-    queryset = queryset.select_related(*select_fields)
-    queryset = queryset.prefetch_related(*prefetch_fields)
-    return queryset
+  def filter_fields(self, related_fields, requested_fields):
+    """
+    Filters'related_fields' according to whether they are included in 
+    'requested_fields'. Used to remove fields from eager loading if they are
+    not requested (and thus not returned by API), avoiding unnecessary DB calls
+    """
+    if not any(requested_fields):
+      return related_fields
+    return [
+      related_field for related_field in related_fields
+      if any(related_field == req or related_field.startswith(req + '__') for req in requested_fields)
+    ]