mysql – 恢复迁移时的ActiveRecord :: IrreversibleMigration异常

我创建了以下Active Record Migration,它添加和删除了一些索引.

class FixIndexes < ActiveRecord::Migration
  def change
    add_index :table1, :field1, :unique => true

    remove_index :table2, :name => "index_table2_on_field1"
    remove_index :table2, :name => "index_table2_on_field2"

    remove_index :table3, :name => "index_table3_on_field1"
    add_index :table3, [:field1, :field2]
  end
end

当我运行迁移($bundle exec rake db：migrate)时,它可以正常工作.

不幸的是,当我尝试恢复迁移($bundle exec rake db：rollback)时,它不起作用并引发ActiveRecord :: IrreversibleMigration异常

==  FixIndexes: reverting =====================================================
rake aborted!
An error has occurred, all later migrations canceled:

ActiveRecord::IrreversibleMigration/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activerecord-3.2.14/lib/active_record/migration/command_recorder.rb:42:in `block in inverse'
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activerecord-3.2.14/lib/active_record/migration/command_recorder.rb:40:in `map'

我的问题是：

>为什么这是不可逆转的迁移？它只是添加和
删除一些索引,而不是数据.
>如果我使用def self.up和def self.down,它将解决问题
而不是改变？
>如何在不添加和删除的情况下立即恢复此更改
从MySQL手动索引？

解决方法:

在Rails中只有少数命令是可逆的(没有手动命令).他们是

> add_column
> add_index
> add_timestamps
> create_table
> create_join_table
> remove_timestamps
> rename_column
> rename_index
> rename_table

参见here

您的迁移包含remove_index,CommandRecorder for Rollback不支持该删除.

如果你查看这个Documentation

Some transformations are destructive in a manner that cannot be
reversed. Migrations of that kind should raise an
ActiveRecord::IrreversibleMigration exception in their down method.

你的情况就是这样,显然会导致ActiveRecord :: IrreversibleMigration异常.

长话短说：如果没有手动命令,remove_index无法撤消.

标签：ruby-on-rails,activerecord,mysql,ruby-on-rails-3,rails-migrations
来源： https://codeday.me/bug/20190831/1772506.html