call

# File activerecord/lib/active_record/migration.rb, line 586
      def call(env)
        @mutex.synchronize do
          @watcher ||= build_watcher do
            @needs_check = true
            ActiveRecord::Migration.check_pending!(connection)
            @needs_check = false
          end

          if @needs_check
            @watcher.execute
          else
            @watcher.execute_if_updated
          end
        end

        @app.call(env)
      end