NoMethodError（未定義的方法`created_at」的零：NilClass）

Question

我在我的conversations_controller以下Rails代碼：

用戶可以是：master或rookie。

@conversations = current_user.master.conversations.all 
@conversations = @conversations.sort_by { |c| c.messages.last.created_at }.reverse 

但在運行此我得到了一個created_atno method error。 如果我做了puts這樣的：

puts "Sorted: #{@conversations.map { |c| c.messages.last}}" 

這給了我以下響應：

Sorted: [#<Message id: 9, content: "some content", user_id: 3, conversation_id: 1, created_at: "2017-03-01 00:00:36", updated_at: "2017-03-01 00:00:36", attachment_url: []>, nil, #<Message id: 11, content: "new message", user_id: 3, conversation_id: 6, created_at: "2017-03-01 15:15:58", updated_at: "2017-03-01 15:15:58", attachment_url: []>] 

如果我得到created_at那麼爲什麼我不能提取最後的消息created_At比較和分類？

在做一個簡單的

puts "The conversations sorted are: #{@conversations.map { |c| c.messages}}" 

給了我以下響應：

The conversations sorted are: [#<ActiveRecord::Associations::CollectionProxy [#<Message id: 1, content: "Hey there! This is my first message.", user_id: 1, conversation_id: 1, created_at: "2017-02-28 23:57:46", updated_at: "2017-02-28 23:57:46", attachment_url: []>, #<Message id: 2, content: "Thank you for your message. I am a coach.", user_id: 3, conversation_id: 1, created_at: "2017-02-28 23:57:46", updated_at: "2017-02-28 23:57:46", attachment_url: []>, #<Message id: 9, content: "adsdadasd", user_id: 3, conversation_id: 1, created_at: "2017-03-01 00:00:36", updated_at: "2017-03-01 00:00:36", attachment_url: ["https://ace-up-www.s3.amazonaws.com/message-attachments%2Fbd450b1e-f85c-47ec-94c7-c539809f8d68%2Frecommendation-bg.jpg"]>]>, #<ActiveRecord::Associations::CollectionProxy []>, #<ActiveRecord::Associations::CollectionProxy [#<Message id: 10, content: "Add a new message", user_id: 8, conversation_id: 6, created_at: "2017-03-01 14:58:29", updated_at: "2017-03-01 14:58:29", attachment_url: []>, #<Message id: 11, content: "new message", user_id: 3, conversation_id: 6, created_at: "2017-03-01 15:15:58", updated_at: "2017-03-01 15:15:58", attachment_url: []>]>] 

正如你可以看到有一個空#<ActiveRecord::Associations::CollectionProxy []>這可能是越來越轉換爲nil。 任何幫助調試呢？

Answer 1

當您嘗試獲取會話最後一條消息的實際沒有任何消息的創建屬性時發生錯誤。

@conversations = current_user.master.conversations.joins(:messages) 
@conversations = @conversations.sort_by { |c| c.messages.last.created_at }.reverse 

通過加入與對話只有ATLEAST一個消​​息會話中的信息將被檢索，因此由我認爲這個問題得到解決。