Fix an N+1 in avatar URLs

This is tricky: the query was being run in
`ObjectStorage::Extension::RecordsUploads#retrieve_from_store!`, but we can't
just add batch loading there, because the `#upload=` method there would use the
result immediately, making the batch only have one item.

Instead, we can pre-emptively add an item to the batch whenever an avatarable
object is initialized, and then reuse that batch item in
`#retrieve_from_store!`. However, this also has problems:

1. There is a lot of logic in `Avatarable#retrieve_upload_from_batch`.
2. Some of that logic constructs a 'fake' model for the batch key. This should
   be fine, because of ActiveRecord's override of `#==`, but it relies on that
   staying the same.

1 files changed, 4 insertions, 0 deletions

diff --git a/app/models/note.rb b/app/models/note.rb
index 02f7a9b1e4f..41c04ae0571 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -435,6 +435,10 @@ class Note < ActiveRecord::Base
     super.merge(noteable: noteable)
   end
 
+  def retrieve_upload(_identifier, paths)
+    Upload.find_by(model: self, path: paths)
+  end
+
   private
 
   def keep_around_commit