update

# File activerecord/lib/active_record/locking/optimistic.rb, line 67
        def update(attribute_names = @attributes.keys) #:nodoc:
          return super unless locking_enabled?
          return 0 if attribute_names.empty?

          lock_col = self.class.locking_column
          previous_lock_value = send(lock_col).to_i
          increment_lock

          attribute_names += [lock_col]
          attribute_names.uniq!

          begin
            relation = self.class.unscoped

            stmt = relation.where(
              relation.table[self.class.primary_key].eq(id).and(
                relation.table[lock_col].eq(quote_value(previous_lock_value))
              )
            ).arel.compile_update(arel_attributes_values(false, false, attribute_names))

            affected_rows = connection.update stmt

            unless affected_rows == 1
              raise ActiveRecord::StaleObjectError.new(self, "update")
            end

            affected_rows

          # If something went wrong, revert the version.
          rescue Exception
            send(lock_col + '=', previous_lock_value)
            raise
          end
        end