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/post_timing_spec.rb

@@ -184,4 +184,41 @@ describe PostTiming do
       expect(post.reload.reads).to eq initial_read_count
     end
   end
+
+  describe '.destroy_last_for' do
+    it 'updates first unread for a user correctly when topic is public' do
+      post = Fabricate(:post)
+      post.topic.update!(updated_at: 10.minutes.ago)
+      PostTiming.process_timings(post.user, post.topic_id, 1, [[post.post_number, 100]])
+
+      PostTiming.destroy_last_for(post.user, post.topic_id)
+
+      expect(post.user.user_stat.reload.first_unread_at).to eq_time(post.topic.updated_at)
+    end
+
+    it 'updates first unread for a user correctly when topic is a pm' do
+      post = Fabricate(:private_message_post)
+      post.topic.update!(updated_at: 10.minutes.ago)
+      PostTiming.process_timings(post.user, post.topic_id, 1, [[post.post_number, 100]])
+
+      PostTiming.destroy_last_for(post.user, post.topic_id)
+
+      expect(post.user.user_stat.reload.first_unread_pm_at).to eq_time(post.topic.updated_at)
+    end
+
+    it 'updates first unread for a user correctly when topic is a group pm' do
+      topic = Fabricate(:private_message_topic, updated_at: 10.minutes.ago)
+      post = Fabricate(:post, topic: topic)
+      user = Fabricate(:user)
+      group = Fabricate(:group)
+      group.add(user)
+      topic.allowed_groups << group
+      PostTiming.process_timings(user, topic.id, 1, [[post.post_number, 100]])
+
+      PostTiming.destroy_last_for(user, topic.id)
+
+      expect(GroupUser.find_by(user: user, group: group).first_unread_pm_at)
+        .to eq_time(post.topic.updated_at)
+    end
+  end
 end