subquery_for

# File activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb, line 735
      def subquery_for(key, select)
        subsubselect = select.clone
        subsubselect.projections = [key]

        # Materialize subquery by adding distinct
        # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
        subsubselect.distinct unless select.limit || select.offset || select.orders.any?

        subselect = Arel::SelectManager.new(select.engine)
        subselect.project Arel.sql(key.name)
        subselect.from subsubselect.as('__active_record_temp')
      end