[Glitch] Fix n+1 query in settings migration

Port 0ad2413b35 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

db/migrate/20230215074424_move_glitch_user_settings.rb

@@ -31,9 +31,11 @@ class MoveGlitchUserSettings < ActiveRecord::Migration[6.1]
   end
 
   def up
-    User.find_each do |user|
+    User.find_in_batches do |users|
-      previous_settings = LegacySetting.where(thing_type: 'User', thing_id: user.id).index_by(&:var)
+      previous_settings_for_batch = LegacySetting.where(thing_type: 'User', thing_id: users.map(&:id)).group_by(&:thing_id)
 
+      users.each do |user|
+        previous_settings = previous_settings_for_batch[user.id]&.index_by(&:var) || {}
         user_settings = Oj.load(user.settings || '{}')
         user_settings.delete('theme')
 
@@ -54,6 +56,7 @@ class MoveGlitchUserSettings < ActiveRecord::Migration[6.1]
         user.update_column('settings', Oj.dump(user_settings)) # rubocop:disable Rails/SkipsModelValidations
       end
     end
+  end
 
   def down; end
 end