PERF: Keep track of first unread PM and first unread group PM for user.

This optimization helps to filter away topics so that the joins on
related tables when querying for unread messages is not expensive.

spec/models/group_user_spec.rb

@@ -160,4 +160,64 @@ describe GroupUser do
       expect(TagUser.lookup(user, :watching_first_post).pluck(:tag_id)).to eq([tag4.id])
     end
   end
+
+  describe '#ensure_consistency!' do
+    fab!(:group) { Fabricate(:group) }
+
+    fab!(:pm_post) { Fabricate(:private_message_post) }
+
+    fab!(:pm_topic) do
+      pm_post.topic.tap { |t| t.allowed_groups << group }
+    end
+
+    fab!(:user) do
+      Fabricate(:user, last_seen_at: Time.zone.now).tap do |u|
+        group.add(u)
+
+        TopicUser.change(u.id, pm_topic.id,
+          notification_level: TopicUser.notification_levels[:tracking],
+          last_read_post_number: pm_post.post_number
+        )
+      end
+    end
+
+    # User that is not tracking topic
+    fab!(:user_2) do
+      Fabricate(:user, last_seen_at: Time.zone.now).tap do |u|
+        group.add(u)
+
+        TopicUser.change(u.id, pm_topic.id,
+          notification_level: TopicUser.notification_levels[:regular],
+          last_read_post_number: pm_post.post_number
+        )
+      end
+    end
+
+    # User that has not been seen
+    fab!(:user_3) do
+      Fabricate(:user).tap do |u|
+        group.add(u)
+
+        TopicUser.change(u.id, pm_topic.id,
+          notification_level: TopicUser.notification_levels[:tracking],
+          last_read_post_number: pm_post.post_number
+        )
+      end
+    end
+
+    it 'updates first unread pm timestamp correctly' do
+      freeze_time 10.minutes.from_now
+
+      post = create_post(
+        user: pm_topic.user,
+        topic_id: pm_topic.id
+      )
+
+      GroupUser.ensure_consistency!
+
+      expect(group.group_users.find_by(user_id: user.id).first_unread_pm_at).to eq_time(post.topic.updated_at)
+      expect(group.group_users.find_by(user_id: user_2.id).first_unread_pm_at).to_not eq_time(post.topic.updated_at)
+      expect(group.group_users.find_by(user_id: user_3.id).first_unread_pm_at).to_not eq_time(post.topic.updated_at)
+    end
+  end
 end