PERF: Only load the current user's topic_user for bookmarks list (#17873)

Previously, for every bookmarked topic, all topic_user records were being preloaded. Only the current user's record is actually required.

This commit introduces a new `perform_custom_preload!` API which bookmarkables can use to add custom preloading logic. We use this in topic_bookmarkable to load just the topic_user data we need (in the same way as `topic_list.rb`).

Co-authored-by: Blake Erickson <o.blakeerickson@gmail.com>

app/serializers/user_post_topic_bookmark_base_serializer.rb

@@ -15,7 +15,6 @@ class UserPostTopicBookmarkBaseSerializer < UserBookmarkBaseSerializer
              :archived,
              :archetype,
              :highest_post_number,
-             :last_read_post_number,
              :bumped_at,
              :slug
 
@@ -51,10 +50,6 @@ class UserPostTopicBookmarkBaseSerializer < UserBookmarkBaseSerializer
     scope.is_whisperer? ? topic.highest_staff_post_number : topic.highest_post_number
   end
 
-  def last_read_post_number
-    topic_user&.last_read_post_number
-  end
-
   def bumped_at
     topic.bumped_at
   end
@@ -62,10 +57,4 @@ class UserPostTopicBookmarkBaseSerializer < UserBookmarkBaseSerializer
   def slug
     topic.slug
   end
-
-  private
-
-  def topic_user
-    @topic_user ||= topic.topic_users.find { |tu| tu.user_id == scope.user.id }
-  end
 end

app/serializers/user_topic_bookmark_serializer.rb

@@ -4,6 +4,9 @@ class UserTopicBookmarkSerializer < UserPostTopicBookmarkBaseSerializer
   # it does not matter what the linked post number is for topic bookmarks,
   # on the client we always take the user to the last unread post in the
   # topic when the bookmark URL is clicked
+
+  attributes :last_read_post_number
+
   def linked_post_number
     1
   end
@@ -63,9 +66,17 @@ class UserTopicBookmarkSerializer < UserPostTopicBookmarkBaseSerializer
     end
   end
 
+  def last_read_post_number
+    topic_user&.last_read_post_number
+  end
+
   private
 
   def topic
     object.bookmarkable
   end
+
+  def topic_user
+    topic.user_data
+  end
 end