compile_update

# File activerecord/lib/arel/crud.rb, line 17
    def compile_update(values, key = nil)
      um = UpdateManager.new(source)
      um.set(values)
      um.take(limit)
      um.offset(offset)
      um.order(*orders)
      um.wheres = constraints
      um.comment(comment)
      um.key = key

      um.ast.groups = @ctx.groups
      @ctx.havings.each { |h| um.having(h) }
      um
    end