refactor: refactor Course.retrieve to prevent calling get_object twice (#4219)

* refactor: refactor Course.retrieve to prevent calling get_object twice

* fix: refetch entitlements if they are created

Author: zawan-ila

course_discovery/apps/api/v1/tests/test_views/test_courses.py

@@ -103,7 +103,7 @@ def test_get(self):
         """ Verify the endpoint returns the details for a single course. """
         url = reverse('api:v1:course-detail', kwargs={'key': self.course.key})
 
-        with self.assertNumQueries(43, threshold=3):
+        with self.assertNumQueries(26, threshold=3):
             response = self.client.get(url)
         assert response.status_code == 200
         assert response.data == self.serialize_course(self.course)
@@ -112,7 +112,7 @@ def test_get_uuid(self):
         """ Verify the endpoint returns the details for a single course with UUID. """
         url = reverse('api:v1:course-detail', kwargs={'key': self.course.uuid})
 
-        with self.assertNumQueries(44):
+        with self.assertNumQueries(27):
             response = self.client.get(url)
         assert response.status_code == 200
         assert response.data == self.serialize_course(self.course)
@@ -121,7 +121,7 @@ def test_get_exclude_deleted_programs(self):
         """ Verify the endpoint returns no deleted associated programs """
         ProgramFactory(courses=[self.course], status=ProgramStatus.Deleted)
         url = reverse('api:v1:course-detail', kwargs={'key': self.course.key})
-        with self.assertNumQueries(43):
+        with self.assertNumQueries(26):
             response = self.client.get(url)
         assert response.status_code == 200
         assert response.data.get('programs') == []
@@ -134,7 +134,7 @@ def test_get_include_deleted_programs(self):
         ProgramFactory(courses=[self.course], status=ProgramStatus.Deleted)
         url = reverse('api:v1:course-detail', kwargs={'key': self.course.key})
         url += '?include_deleted_programs=1'
-        with self.assertNumQueries(47):
+        with self.assertNumQueries(29):
             response = self.client.get(url)
         assert response.status_code == 200
         assert response.data == self.serialize_course(self.course, extra_context={'include_deleted_programs': True})

course_discovery/apps/api/v1/views/courses.py

@@ -516,8 +516,12 @@ def retrieve(self, request, *args, **kwargs):
         course = self.get_object()
         if get_query_param(request, 'editable') and not course.entitlements.exists():
             create_missing_entitlement(course)
-
-        return super().retrieve(request, *args, **kwargs)
+            course.refresh_from_db(fields=['entitlements'])
+        # Rather than call super().retrieve, we instantiate the serializer and return its
+        # data ourselves. This is to prevent duplicate calls (and hence duplicate queries)
+        # to self.get_object. Note that we have called get_object once already(see above).
+        serializer = self.get_serializer(course)
+        return Response(serializer.data)
 
 
 class CourseRecommendationViewSet(RetrieveModelMixin, viewsets.GenericViewSet):