如何重命名列并同时通过迁移更改其类型 [英] How to rename a column and change its type by migration same time

问题描述

在我的 general_exams 表中，有一列名为 semester，类型为 string.现在我想把它的名字改成semester_id，类型是integer.我已经阅读了关于迁移的内容，并且它有可用的转换:

In my general_exams table, I have a column named semester, type is string. Now I want to change its name to semester_id, type is integer. I have read about migration and it has available transformations:

• rename_column(table_name, column_name, new_column_name):重命名列但保留类型和内容.
• change_column(table_name, column_name, type, options):使用与 add_column 相同的参数将列更改为不同的类型.

所以，我像这样创建我的迁移文件:

So, I create my migration file like this:

class RenameSemesterFromGeneralExams < ActiveRecord::Migration

  def change
    rename_column :general_exams, :semester, :semester_id
    change_column :general_exams, :semester_id, :integer
  end
end

但是，当我运行 rake db:migrate 时，它有错误:

But, when I run rake db:migrate, it has error:

==  RenameSemesterFromGeneralExams: migrating =================================
-- rename_column(:general_exams, :semester, :semester_id)
   -> 0.0572s
-- change_column(:general_exams, :semester_id, :integer)
rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  column "semester_id" cannot be cast to type integer
: ALTER TABLE "general_exams" ALTER COLUMN "semester_id" TYPE integer

在我的表 GeneralExam 中，我销毁了所有数据.那么，任何人都可以告诉我我该怎么做?或者我必须创建两个迁移文件?

In my table GeneralExam, I destroyed all data. So, anyone can tell me how can I do that? Or I must create two migration files?

推荐答案

您的问题可能是 semester 包含无法转换为 integers 的数据.这就是您收到强制转换错误的原因.

Your problem is probably that the semester contains data that cannot be converted to integers. That's why you get a cast error.

我怀疑您需要做更多的工作来完成这项工作，因为唯一想到的是删除列并创建一个具有正确值的新列.

I suspect you need to do more work to make this work as the only thing that comes to mind is removing the column and creating a new one with the correct values.

但是您可以简单地在一次迁移中先 remove_column 和 add_column .这应该可以完美运行.

But you can simply remove_column and then add_column in one migration. That should work flawlessly.

我还建议您先只添加_column，然后进行映射过程，将旧的 semester 值映射到新的 semester_id，然后删除该列.

I'd also suggest you only add_column first, then do the mapping process where you map the old semester value onto the new semester_id and then drop the column.

请记住，您可以在迁移中执行 ActiveRecord 操作.所以你可以把代码放在那里.

Keep in mind that you can do ActiveRecord manipulations inside your migration. So you can put that code in there.

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