如何使用純活動記錄（rails）執行此sql

Question

我無法弄清楚如何純粹用活動記錄進行管理。我的理解是不處理外部聯接...

SQL：

SELECT * FROM people 
LEFT OUTER JOIN responses 
ON people.id = responses.person_id 
WHERE responses.person_id is NULL; 

機型：

Person 
id 
has_many :responses 

Response 
id 
belongs_to :person 

有什麼建議？

Answer 1

named_scope :without_responses, :include => :responses, 
           :conditions => "responses.id IS NULL"