迁移不会回滚

Question

我已经运行这个迁移：

class AddUniqueToLocationColumnName < ActiveRecord::Migration 
    def change 
    remove_index :locations, :name 
    add_index :locations, :name, unique: true 
    end 
end 

，现在我想回滚，但其示值误差：

StandardError: An error has occurred, this and all later migrations canceled: remove_index is only reversible if given a :column option.

我怎么可以回滚此迁移到我以前的版本？

Answer 1

尽量明确定义上下：

class AddUniqueToLocationColumnName < ActiveRecord::Migration 
    def self.up 
    remove_index :locations, column: :name 
    add_index :locations, :name, unique: true 
    end 

    def self.down 
    remove_index :locations, column: :name # remove unique index 
    add_index :locations, :name # adds just index, without unique 
    end 
end