In the simple case, MySQL allows us to place JOINs directly into the UPDATE query. However, this does not allow for LIMIT, OFFSET and ORDER. To support these, we must use a subquery. However, MySQL is too stupid to create a temporary table for this automatically, so we have to give it some prompting in the form of a subsubquery. Ugh!
# File activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb, line 606 def join_to_update(update, select) #:nodoc: if select.limit || select.offset || select.orders.any? subsubselect = select.clone subsubselect.projections = [update.key] subselect = Arel::SelectManager.new(select.engine) subselect.project Arel.sql(update.key.name) subselect.from subsubselect.as('__active_record_temp') update.where update.key.in(subselect) else update.table select.source update.wheres = select.constraints end end