Contains help about designing tables by adding and removing columns and changing column properties.
包含有关通过添加和移除列以及更改列属性来设计表的帮助信息。
Most schema manipulations, such as adding and removing columns, can be done using Rails's built-in methods.
大多数的模式操作，比如添加和删除列，都可以使用Rails的内置方法完成。
When a user is viewing a report they may wish to customize the report by removing columns and subsequently adding the columns back to the report.
用户在查看报告时，可能希望先去掉一些列，然后再将这些列加回到报告中。
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