在Rails移植中使用模型關聯的問題

Question

我正在嘗試修復向評論表添加列並執行一些更新的一些問題。但有一整天此錯誤：

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

You have a nil object when you didn't expect it! You might have expected an instance of ActiveRecord::Base. The error occurred while evaluating nil.[]

遷移代碼：

class AddCommenterNameToComments < ActiveRecord::Migration 
    def self.up 
    add_column :comments, :commenter_name, :string 
    Comment.reset_column_information 

    #to update all comments with commenter name 
    Comment.all.each do |comment| 
     unless comment.is_system_message? 
      comment.update_attribute(:commenter_name, comment.user.name) 
     end 
    end 
    end 

    def self.down 
    remove_column :comments, :commenter_name 
    end 
end 

請幫助。

Answer 1

該關聯可能不是問題，而是測試它是否可以通過從註釋表中傳入user_id列來搜索用戶對象。因此，舉例來說： 編輯與OP建議修復

class AddCommenterNameToComments < ActiveRecord::Migration 
    class Comment < ActiveRecord::Base 
    end 
    class User < ActiveRecord::Base 
    end 
    def self.up 
    add_column :comments, :commenter_name, :string 
    Comment.reset_column_information 

    #to update all comments with commenter name 
    Comment.all.each do |comment| 
     unless comment.is_system_message? 
     user = User.find_by_id(comment.user_id) 
     if user 
      comment.update_attribute(:commenter_name, user.name) 
     else 
      comment.update_attribute(:commenter_name, "User deleted") 
     end 
     end 
    end 
    end 

    def self.down 
    remove_column :comments, :commenter_name 
    end 
end 

現在說，這個問題可能是你在意見表不再對應於用戶表用戶對象爲user_id。你可能也想檢查一下，以防結果不成問題。