Fix N+1 query problem in GET /api/v1/changesets/ (#744)

The DRF Serializer for the Changeset model was using obj.tags.filter()
and obj.reasons.filter() to filter the lists of reasons and tags
(only showing enabled ones to non-staff users). This had the probably
unexpected consequence of generating an additional two database
queries for each changeset in the resulting list. The reasons and tags
related fields were being prefetched, but prefetching only works for
iterating the entire relation.

The fix for this problem is straightforward: instead of using .filter(),
we can just filter the reasons and tags in Python. This allows the
prefetching to work as intended and avoids the additional queries.

Author: jake-low

osmchadjango/changeset/serializers.py

@@ -56,18 +56,17 @@ class ChangesetSerializer(ChangesetSerializerToStaff):
     tags = SerializerMethodField()
 
     def get_reasons(self, obj):
-        return BasicSuspicionReasonsSerializer(
-            obj.reasons.filter(is_visible=True),
-            many=True,
-            read_only=True
-            ).data
+        # Using obj.reasons.filter() would generate a new query, which in the context
+        # of ChangesetListAPIView would waste the prefetching we've done and cause
+        # N+1 total queries. To avoid this, we instead filter the reasons in Python.
+        visible_reasons = [reason for reason in obj.reasons.all() if reason.is_visible]
+        return BasicSuspicionReasonsSerializer(visible_reasons, many=True, read_only=True).data
 
     def get_tags(self, obj):
-        return BasicTagSerializer(
-            obj.tags.filter(is_visible=True),
-            many=True,
-            read_only=True
-            ).data
+        # Filter the tags in Python rather than using obj.tags.filter() which would
+        # generate a new query (see above)
+        visible_tags = [tag for tag in obj.tags.all() if tag.is_visible]
+        return BasicTagSerializer(visible_tags, many=True, read_only=True).data
 
 
 class UserWhitelistSerializer(ModelSerializer):