How to find all records with zero number of has_many assocations?

问题

Let's say you have Question model with following has_many association:(example taken from this plugin)

has_many :comment_threads, 
         :class_name => "Comment", 
         :as         => :commentable, 
         :dependent  => :destroy

How would I define a scope or a class method that returns questions that has no associated comments?

Basically I want Question.unanswered to return all questions with zero comments.

回答1:

I think approach with counter_cache is nicer and faster, but you can create the scope you want like that (you might need some adjustments if I guessed tables or columns names wrong):

scope :unanswered, 
  joins('LEFT OUTER JOIN comments ON comments.commentable_id = questions.id').
  where('comments.id IS NULL')

Using LEFT OUTER JOIN generates joined table where for an uncommented question all columns of comments table are set to NULL. These are exactly the rows we need so we filter them using where.

来源：https://stackoverflow.com/questions/8985187/how-to-find-all-records-with-zero-number-of-has-many-assocations