PERF: Updating first unread PM for user not respecting limits. (#15056)

In b8c8909a9d, we introduced a regression
where users may have had their `UserStat.first_unread_pm_at` set
incorrectly. This commit introduces a migration to reset `UserStat.first_unread_pm_at` back to
`User#created_at`.

Follow-up to b8c8909a9d.

app/models/user_stat.rb

@@ -53,6 +53,14 @@ class UserStat < ActiveRecord::Base
         )
         GROUP BY tau.user_id
       ) AS X ON X.user_id = u1.id
+      WHERE u1.id IN (
+        SELECT id
+        FROM users
+        WHERE last_seen_at IS NOT NULL
+        AND last_seen_at > :last_seen
+        ORDER BY last_seen_at DESC
+        LIMIT :limit
+      )
     ) AS Z
     WHERE us.user_id = Z.user_id
     SQL

db/migrate/20211123033311_reset_first_unread_pm_at_on_user_stat.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class ResetFirstUnreadPmAtOnUserStat < ActiveRecord::Migration[6.1]
+  def up
+    execute <<~SQL
+    UPDATE user_stats us
+    SET first_unread_pm_at = u.created_at
+    FROM users u
+    WHERE u.id = us.user_id
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

spec/models/user_stat_spec.rb

@@ -90,6 +90,8 @@ describe UserStat do
     end
 
     it 'updates first unread pm timestamp correctly' do
+      freeze_time
+
       pm_topic = Fabricate(:private_message_topic)
       user = pm_topic.user
       user.update!(last_seen_at: Time.zone.now)
@@ -107,26 +109,57 @@ describe UserStat do
         notification_level: TopicUser.notification_levels[:regular]
       )
 
-      # User that has not been seen
-      user_3 = Fabricate(:user)
+      # User that has not been seen recently
+      user_3 = Fabricate(:user, last_seen_at: 1.year.ago)
       pm_topic.allowed_users << user_3
 
       TopicUser.change(user_3.id, pm_topic.id,
         notification_level: TopicUser.notification_levels[:tracking]
       )
 
-      freeze_time 10.minutes.from_now
+      user_3_orig_first_unread_pm_at = user_3.user_stat.first_unread_pm_at
 
-      post = create_post(
-        user: Fabricate(:admin),
-        topic_id: pm_topic.id
+      # User that is not related to the PM
+      user_4 = Fabricate(:user, last_seen_at: Time.zone.now)
+      user_4_orig_first_unread_pm_at = user_4.user_stat.first_unread_pm_at
+
+      # User for another PM topic
+      pm_topic_2 = Fabricate(:private_message_topic)
+      user_5 = pm_topic_2.user
+      user_5.update!(last_seen_at: Time.zone.now)
+      create_post(user: user_5, topic_id: pm_topic_2.id)
+
+      TopicUser.change(user_5.id, pm_topic_2.id,
+        notification_level: TopicUser.notification_levels[:tracking]
       )
 
-      UserStat.ensure_consistency!
+      # User out of last seen limit
+      user_6 = Fabricate(:user, last_seen_at: Time.zone.now)
+      pm_topic.allowed_users << user_6
+      create_post(user: user_6, topic_id: pm_topic.id)
 
-      expect(user.user_stat.reload.first_unread_pm_at).to eq_time(post.topic.updated_at)
-      expect(user_2.user_stat.reload.first_unread_pm_at).to_not eq_time(post.topic.updated_at)
-      expect(user_3.user_stat.reload.first_unread_pm_at).to_not eq_time(post.topic.updated_at)
+      TopicUser.change(user_6.id, pm_topic.id,
+        notification_level: TopicUser.notification_levels[:tracking]
+      )
+
+      user_6_orig_first_unread_pm_at = user_6.user_stat.first_unread_pm_at
+
+      admin = Fabricate(:admin)
+      create_post(user: admin, topic_id: pm_topic.id)
+      create_post(user: admin, topic_id: pm_topic_2.id)
+      pm_topic.update!(updated_at: 10.minutes.from_now)
+      pm_topic_2.update!(updated_at: 20.minutes.from_now)
+
+      stub_const(UserStat, "UPDATE_UNREAD_USERS_LIMIT", 4) do
+        UserStat.ensure_consistency!
+      end
+
+      expect(user.user_stat.reload.first_unread_pm_at).to eq_time(pm_topic.reload.updated_at)
+      expect(user_2.user_stat.reload.first_unread_pm_at).to eq_time(UserStat::UPDATE_UNREAD_MINUTES_AGO.minutes.ago)
+      expect(user_3.user_stat.reload.first_unread_pm_at).to eq_time(user_3_orig_first_unread_pm_at)
+      expect(user_4.user_stat.reload.first_unread_pm_at).to eq_time(UserStat::UPDATE_UNREAD_MINUTES_AGO.minutes.ago)
+      expect(user_5.user_stat.reload.first_unread_pm_at).to eq_time(pm_topic_2.reload.updated_at)
+      expect(user_6.user_stat.reload.first_unread_pm_at).to eq_time(user_6_orig_first_unread_pm_at)
     end
   end