Draft: Test N+1 fix

What does this MR do and why?

Describe in detail what your merge request does and why.

This is a temporary MR to test possible fixes to the N+1 issue being seen in a community contribution.

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Numbered steps to set up and validate the change are strongly suggested.

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app/serializers/group_child_entity.rb

@@ -89,7 +89,7 @@ def membership
   # rubocop: enable CodeReuse/ActiveRecord
 
   def project?
-    object.is_a?(Project)
+    object.is_a?(Project) && preload_project_associations
   end
 
   def type
@@ -117,18 +117,14 @@ def last_pipeline
     return unless project? && can?(request.current_user, :read_build, object)
 
     BatchLoader.for(object).batch do |projects, loader|
-      # rubocop: disable CodeReuse/ActiveRecord
-      # This will load the project_feature association for all the projects for the line after next line
-      ActiveRecord::Associations::Preloader.new.preload(projects, :project_feature)
       builds_enabled_projects = projects.select(&:builds_enabled?)
       # Prefill nil for all the projects
       projects.each { |project| loader.call(project, nil) }
 
       # This is to load last_pipeline for all the builds-enabled projects in one SQL
       Ci::Pipeline # The DISTINCT select statement will work with the below ORDER statement to return the last pipeline
-        .select("DISTINCT ON (project_id) *")
-        .where(project_id: builds_enabled_projects.map(&:id)).ci_sources
-        .order(:project_id, id: :desc)
+        .select("DISTINCT ON (project_id) id, project_id, status").ci_sources
+        .where(project_id: builds_enabled_projects.map(&:id)).order(:project_id, id: :desc) # rubocop: disable CodeReuse/ActiveRecord
         # `each` is used instead of `find_each` because `find_each` will overwrite the above ORDER statement that we
         # need for the last pipelines query to work.
         .each do |pipeline|
@@ -141,9 +137,19 @@ def last_pipeline
                       text: pipeline.status
                     })
       end
-      # rubocop: enable CodeReuse/ActiveRecord
     end
   end
+
+  def preload_project_associations
+    @preload_project_associations ||=
+      BatchLoader.for(object).batch do |projects, loader|
+        # This will load the associations for all the projects for the later serialization
+        preloader = ActiveRecord::Associations::Preloader.new
+        preloader.preload(projects, :project_feature) # rubocop:disable CodeReuse/ActiveRecord
+        # NOTE: we just need to fill `nil` to fulfill the loader
+        projects.each { |project| loader.call(project, nil) }
+      end && true
+  end
 end
 
 GroupChildEntity.prepend_mod_with('GroupChildEntity')