Prefetch awesome_count and has_user_awesomed

Previous version caused 2 extra queries for every dweet to generate
"has_user_awesomed" (one user query and then a dweet query) and 1 extra
query to generate the awesome_count.

This commit achieves the same result in the main query instead, using
the prefetch_related mechanism  (at the cost of a more expensive main
query with some JOINs)

Author: lionleaf

dwitter/serializers_v2.py

@@ -141,8 +141,8 @@ class DweetSerializer(serializers.ModelSerializer):
     remix_of = RemixOfSerializer(source='reply_to')
     remixes = serializers.PrimaryKeyRelatedField(read_only=True, many=True)
 
-    awesome_count = serializers.SerializerMethodField()
-    has_user_awesomed = serializers.SerializerMethodField()
+    awesome_count = serializers.IntegerField()
+    has_user_awesomed = serializers.BooleanField()
     author = UserSerializer()
     comments = CommentSerializer(many=True)
 
@@ -160,9 +160,4 @@ class Meta:
             'has_user_awesomed',
         )
 
-    def get_has_user_awesomed(self, dweet):
-        user = self.context['request'].user
-        return dweet.likes.filter(pk=user.pk).exists()
 
-    def get_awesome_count(self, obj):
-        return obj.likes.all().count()

dwitter/views_v2.py

@@ -4,6 +4,7 @@
 from django.contrib.auth.models import User
 
 from django.db.models import Prefetch, Count
+from django.db.models.expressions import Exists, OuterRef
 from django.utils import timezone
 
 from rest_framework import mixins, viewsets, status
@@ -70,18 +71,30 @@ class DweetViewSet(mixins.RetrieveModelMixin,
     queryset = Dweet.objects.all().select_related(
         'author',
     ).prefetch_related(
+        Prefetch('reply_to', queryset=Dweet.objects.select_related('author')),
         'likes',
+        Prefetch('remixes'),
         Prefetch('comments', queryset=Comment.objects.select_related('author'))
+    ).annotate(
+        awesome_count=Count('likes')
     ).order_by('-hotness')
+
     serializer_class = DweetSerializer
 
+    def retrieve(self, request, pk):
+        self.queryset = self.queryset.annotate(
+            has_user_awesomed=Exists(Dweet.objects.filter(
+                id=OuterRef('id'), likes__in=[request.user.id]))
+        )
+
+        return super().retrieve(request, pk)
+
     def create(self, request):
         code = request.data.get('code', '')
         if (length_of_code(code) > 140):
             raise ValidationError("Code longer than 140 characters")
 
         remix_of_pk = request.data.get('remix_of', -1)
-
         # Using filter().first() will return None if it doesn't exist
         # instead of raising a DoesNotExist exception
         remix_of = self.queryset.filter(pk=remix_of_pk).first()
@@ -130,6 +143,11 @@ def list(self, request):
         if order_by == '-awesome_count':
             self.queryset = self.queryset.annotate(awesome_count=Count('likes'))
 
+        self.queryset = self.queryset.annotate(
+            has_user_awesomed=Exists(Dweet.objects.filter(
+                id=OuterRef('id'), likes__in=[request.user.id]))
+        )
+
         self.queryset = self.queryset.order_by(order_by).filter(
             posted__gte=posted_after, posted__lt=posted_before, **filters)