build_change_column_definition

# File activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb, line 408
      def build_change_column_definition(table_name, column_name, type, **options) # :nodoc:
        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

        if options[:collation] == :no_collation
          options.delete(:collation)
        else
          options[:collation] ||= column.collation if text_type?(type)
        end

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

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