method

join_to_update

v3.2.1 - Show latest stable - 0 notes - Class: AbstractMysqlAdapter

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What's this?

join_to_update(update, select) public

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!

Show source

# File activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb, line 290
      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

join_to_update

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