迁移以更改字段的默认值，并仅在具有旧默认值的情况下，将所有现有记录的值更改为新的默认值. [英] Migration to change default value for a field and change all existing record's value to new default value only if it has old default value.

问题描述

我需要将字段的默认值从0更改为3，但是要注意的是我已经有成千上万的记录，并且仅当记录的默认值为0时才希望这些记录将值从0更改为3.其他值，例如1、2，则应保持不变.我该怎么做?

I need to change the default value of a field from 0 to 3, but the catch is i have thousands of records already and want those records to change the value to 3 from 0 only if the record has default value 0 but for other values like 1, 2 it should remain the same. how can i do it?

推荐答案

在迁移中，您应该使用change_column方法更改表设置，如下所示:

In the migration you should use the method change_column to alter the table settings like this:

change_column :my_models, :attribute_name, :integer, :default => 3

然后要更新所有现有记录，而不是遍历所有记录并分别更新它们，可以使用update_all方法，如下所示:

And then to update all existing records, instead of looping through all records and updating them individually you could use the method update_all like this:

MyModel.update_all({ :attribute_name => 3 }, { :attribute_name => 0 })

第一个参数告诉方法要设置什么值，第二个参数告诉它要更新哪些行的条件.

The first argument tells the method what value to set and the second tells it the condition for which rows to update.

这篇关于迁移以更改字段的默认值，并仅在具有旧默认值的情况下，将所有现有记录的值更改为新的默认值.的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！