FIX: Unread group PMs should use `GroupUser#first_unread_pm_at`. (#14075)

This bug was causing unread PMs for groups to appear inaccurate.

lib/topic_query/private_message_lists.rb

@@ -182,9 +182,23 @@ class TopicQuery
         staff: user.staff?
       )
 
-      first_unread_pm_at = UserStat
-        .where(user_id: user.id)
-        .pluck_first(:first_unread_pm_at)
+      first_unread_pm_at =
+        case type
+        when :user
+          user_first_unread_pm_at(user)
+        when :group
+          GroupUser
+            .where(user: user, group: group)
+            .pluck_first(:first_unread_pm_at)
+        else
+          user_first_unread_pm_at = user_first_unread_pm_at(user)
+
+          group_first_unread_pm_at = GroupUser
+            .where(user: user)
+            .minimum(:first_unread_pm_at)
+
+          [user_first_unread_pm_at, group_first_unread_pm_at].compact.min
+        end
 
       if first_unread_pm_at
         list = list.where("topics.updated_at >= ?", first_unread_pm_at)
@@ -246,5 +260,9 @@ class TopicQuery
           .first
       end
     end
+
+    def user_first_unread_pm_at(user)
+      UserStat.where(user: user).pluck_first(:first_unread_pm_at)
+    end
   end
 end

spec/lib/topic_query/private_message_lists_spec.rb

@@ -111,16 +111,26 @@ describe TopicQuery::PrivateMessageLists do
   end
 
   describe '#list_private_messages_all_unread' do
-    it 'returns a list of unread private messages' do
-      topics = TopicQuery.new(nil).list_private_messages_all_unread(user_2).topics
-
-      expect(topics).to eq([])
-
+    before do
       TopicUser.find_by(user: user_2, topic: group_message).update!(
         last_read_post_number: 1
       )
 
       create_post(user: user, topic: group_message)
+    end
+
+    it 'returns a list of unread private messages' do
+      topics = TopicQuery.new(nil).list_private_messages_all_unread(user_2).topics
+
+      expect(topics).to contain_exactly(group_message)
+    end
+
+    it 'takes into account first_unread_pm_at optimization' do
+      user_2.user_stat.update!(first_unread_pm_at: group_message.created_at + 1.day)
+
+      GroupUser.find_by(user: user_2, group: group).update!(
+        first_unread_pm_at: group_message.created_at - 1.day
+      )
 
       topics = TopicQuery.new(nil).list_private_messages_all_unread(user_2).topics