Fix n+1 query in settings migration (#24783)

db/migrate/20230215074423_move_user_settings.rb

@@ -59,9 +59,11 @@ class MoveUserSettings < ActiveRecord::Migration[6.1]
   end
 
   def up
-    User.find_each do |user|
-      previous_settings = LegacySetting.where(thing_type: 'User', thing_id: user.id).index_by(&:var)
+    User.find_in_batches do |users|
+      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     = {}
 
         MAPPING.each do |legacy_key, new_key|
@@ -81,6 +83,7 @@ class MoveUserSettings < ActiveRecord::Migration[6.1]
         user.update_column('settings', Oj.dump(user_settings)) # rubocop:disable Rails/SkipsModelValidations
       end
     end
+  end
 
   def down; end
 end