subquery_for

# File activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb, line 723
        def subquery_for(key, select)
          subselect = select.clone
          subselect.projections = [key]

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

          key_name = quote_column_name(key.name)
          Arel::SelectManager.new(subselect.as("__active_record_temp")).project(Arel.sql(key_name))
        end