serializers: Optimize query performance on SeriesMixin for 'related_series'

Ensure the retrival of series is keept at O(1) complexity.

Closes getpatchwork#506

Signed-off-by: andrepapoti <[email protected]>

Author: andrepapoti

patchwork/api/series.py

@@ -95,7 +95,11 @@ class SeriesMixin(object):
     def get_queryset(self):
         return (
             Series.objects.all()
-            .prefetch_related('patches__project', 'cover_letter__project')
+            .prefetch_related(
+                'patches__project',
+                'cover_letter__project',
+                'related_series__project',
+            )
             .select_related('submitter', 'project')
         )
 

patchwork/tests/api/test_series.py

@@ -152,7 +152,7 @@ def test_list_bug_335(self):
             create_cover(series=series_obj)
             create_patch(series=series_obj)
 
-        with self.assertNumQueries(6):
+        with self.assertNumQueries(7):
             self.client.get(self.api_url())
 
     @utils.store_samples('series-detail')