Draft: Make `finalize_background_migration` to work properly with many DBs
This makes the finalize_background_migration when running migration
class to restore original connections allowing the application
to use ApplicationRecord as a way to access main database.
Running db:migrate will switch ActiveRecord::Base.connection
to be any of the databases depending on a migration context.
This naturally breaks all background migrations that do use ApplicationRecord
as a way to access :main.
The change introduced here is "a workaround" / "a minimal change" to allow finalize to execute background migration that is normally executed in sidekiq context within a migration context.
Different ways are better described here: #358430 (comment 931712686)
Edited by Kamil Trzciński