Merge pull request open5e#647 from calumbell/refactor/eager-loading-mixin

Refactor eager loading code into `EagerLoadingMixin`

Author: calumbell

api_v2/views/mixins.py

@@ -0,0 +1,59 @@
+class EagerLoadingMixin:
+  """
+  Mixin to apply eager loading optimisations to a ViewSet.
+  
+  Dynamically applies `selected_related()` for ForeignKey fields and 
+  `prefetch_related()` from ManyToMany/reverse relationships. This improves 
+  query efficiency and prevents N+1 problems
+  
+  ## Usage
+  1. Make sure your ViewSet inherits from `EagerLoadingMixin` before its base
+  class (ie. ReadOnlyModelViewSet).
+  2. Re-define `select_related_fields` and `prefetch_related_fields` lists on 
+  the child ViewSet to specify relationships to optimise.
+  
+  ## 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
+
+      # ForeignKey relations to optimise with select_related()
+      select_related_fields = []
+      # ManyToMany / reverse relations to optimise with prefetch_related()
+      prefetch_related_fields = []
+  ```
+  """
+
+  # Override these lists in child views 
+  select_related_fields = [] # ForeignKeys to optimise
+  prefetch_related_fields = [] # ManyToMany & reverse relationships to prefetch
+
+  def get_queryset(self):
+    """Override DRF's default get_queryset() method to apply eager loading"""
+    queryset = super().get_queryset()
+    request = self.request
+
+    # Get query parameters
+    requested_fields = request.query_params.get('fields', '')
+    depth = int(request.query_params.get('depth', 0))
+
+    if requested_fields:
+      requested_fields = set(requested_fields.split(','))
+    else:
+      # If no fields requested, apply all opitmisations
+      requested_fields = set(self.select_related_fields + self.prefetch_related_fields)
+
+    # Filter fields based on on which have been requested
+    select_fields = [field for field in self.select_related_fields if field in requested_fields]
+    prefetch_fields = [field for field in self.prefetch_related_fields if field in requested_fields]
+    
+    # Apply optimisations
+    if select_fields:
+      queryset = queryset.select_related(*select_fields)
+    if prefetch_fields:
+      queryset = queryset.prefetch_related(*prefetch_fields)
+
+    return queryset