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如何具体说明在创纪录或更新某个领域时只叫作的“积极记录”中的退约?
原标题:How to specify a callback in activerecord that is only called when the record is created or a certain field is updated?

我正在编写一个小的核对手册,平衡网络应用,教授自己的铁路。 账户有<代码>balance field,我想在相关<代码>Transaction.amount领域发生变化/重新编号时加以更新。 我看着这套书,但我很不时想如何做。

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