change_column_for_alter

# File activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb, line 667
        def change_column_for_alter(table_name, column_name, type, options = {})
          column = column_for(table_name, column_name)
          type ||= column.sql_type

          unless options.key?(:default)
            options[:default] = column.default
          end

          unless options.key?(:null)
            options[:null] = column.null
          end

          unless options.key?(:comment)
            options[:comment] = column.comment
          end

          td = create_table_definition(table_name)
          cd = td.new_column_definition(column.name, type, options)
          schema_creation.accept(ChangeColumnDefinition.new(cd, column.name))
        end