PERF: Make TopicViewSerializer#requested_group_name more efficient. (#14196)

* Avoid executing a query when the custom field doesn't exist
* Avoid generating an ActiveRecord when all we need is the name.

spec/serializers/topic_view_serializer_spec.rb

@@ -484,4 +484,26 @@ describe TopicViewSerializer do
       end
     end
   end
+
+  describe '#requested_group_name' do
+    fab!(:pm) { Fabricate(:private_message_post).topic }
+    fab!(:group) { Fabricate(:group) }
+
+    it 'should return the right group name when PM is a group membership request' do
+      pm.custom_fields[:requested_group_id] = group.id
+      pm.save!
+
+      user = pm.first_post.user
+      group.add_owner(user)
+      json = serialize_topic(pm, user)
+
+      expect(json[:requested_group_name]).to eq(group.name)
+    end
+
+    it 'should not include the attribute for a non group membership request PM' do
+      json = serialize_topic(pm, pm.first_post.user)
+
+      expect(json[:requested_group_name]).to eq(nil)
+    end
+  end
 end