爲什麼發生不可逆遷移？

Question

錯誤說明此代碼不能回滾：

class AddCountToTag < ActiveRecord::Migration 
    def change 
     change_table :tags do |t| 
      t.integer :count 
      t.index :count 
     end 
    end 
end 

哪裏錯了？

Answer 1

change_table尚不支持可逆遷移。 See this comment at the top of the file（引用如下）：

# <tt>ActiveRecord::Migration::CommandRecorder</tt> records commands done during 
    # a migration and knows how to reverse those commands. The CommandRecorder 
    # knows how to invert the following commands: 
    # 
    # * add_column 
    # * add_index 
    # * add_timestamps 
    # * create_table 
    # * create_join_table 
    # * remove_timestamps 
    # * rename_column 
    # * rename_index 
    # * rename_table 

如果您需要能夠扭轉，則可以使用add_column代替change_table：

class AddCountToTag < ActiveRecord::Migration 
    def change 
     add_column :tags, :count, :integer 
     ... 
    end 
end