Rails迁移:self.up和self.down与变更 [英] Rails migrations: self.up and self.down versus change

问题描述

看起来新的rails版本相对于self.up和self.down方法具有更改"功能.

Looks like the new rails version has "change" versus self.up and self.down methods.

因此，当必须回滚迁移时会发生什么，它如何知道要执行的操作.我需要基于在线教程实现以下方法:

So what happens when one has to roll back a migration how does it know what actions to perform. I have the following method that I need to implement based on an online tutorial:

class AddImageToUsers < ActiveRecord::Migration
  def self.up
    add_column :users, :image_file_name, :string
    add_column :users, :image_content_type, :string
    add_column :users, :image_file_size, :integer
    add_column :users, :image_updated_at, :datetime
  end

  def self.down
    remove_column :users, :image_file_name, :string
    remove_column :users, :image_content_type, :string
    remove_column :users, :image_file_size, :integer
    remove_column :users, :image_updated_at, :datetime
  end    
end

如何使用新的更改方法来做同样的事情?

How can I do the same using the new change method?

推荐答案

对于许多操作 rails可以猜测什么是逆操作(没有问题).例如，在您的情况下，回滚时调用add_column的反向操作是什么?当然是remove_column. create_table的逆是什么? drop_table.因此，在这些情况下，rails知道如何回滚和定义down方法是多余的(您可以在文档中看到

For many operations rails can guess what is the inverse operation (without problems). For example, in your case what is the reverse operation of add_column to call when you rollback? Of course it's remove_column. What is the inverse of create_table? It's drop_table. So in these cases rails know how to rollback and define a down method is superfluous (you can see in the documentation the methods currently supported from the change method).

但是要注意，因为对于某种操作，您仍然需要定义down方法，例如，如果您更改小数列的精度，那么如何猜测回滚时的原始精度?这是不可能的，因此您需要定义down方法.

But pay attention because for some kind of operation you still need to define the down method, for example if you change the precision of a decimal column how to guess the original precision on rollback? It's not possible, so you need to define the down method.

如上所述，建议您阅读《 Rails迁移指南》 .

As said, I suggest you to read the Rails Migrations Guide.

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