ruby on rails - Get last comments created -

there articles , comments.

the goal: list comments posted last on articles.

for example:

article #1 has 3 comments

article #2 has 1 comment

article #3 has no comments

---

goal: 2 comments created last on article #1, #2

class article < activerecord::base   has_many :comments end  class comment < activerecord::base   belongs_to :article, counter_cache: true   def self.last_comment    # list comments created last on article  end end   class commentscontroller < applicationcontroller  def last_comments   @last_comments = comment.last_comment end  end 

one way cache last_comment_id on article.

class article < activerecord::base   has_many :comments, after_add: :update_last_comment, after_remove: :update_last_comment   has_one :last_comment, class_name: "comment"    private    # process asynchronous job   def update_last_comment     self.last_comment = comments.order(created_at: :desc).first     self.save!   end end  class comment < activerecord::base   belongs_to :article, counter_cache: true    def self.last_comment     joins(:article).       where("id = articles.last_comment_id")   end end 

the after_add/after_remove activerecord association callbacks, documented here.