按最新创建的评论排序

Question

What i have created is a "active" field in my topics table which i can use to display the active topics , which will contain at first the time the topic was created and when someone comments it will use the comment.created_at time and put it in the active field in the topics table, like any other forum system.

I found i similar question here How to order by the date of the last comment and sort by last created otherwise?

But it wont work for me, im not sure why it wouldn't. And i also don't understand if i need to use counter_cache in this case or not. Im using a polymorphic association for my comments, so therefore im not sure how i would use counter_cache. It works fine in my topic table to copy the created_at time to the active field. But it wont work when i create a comment.

Error:

NoMethodError in CommentsController#create

undefined method `topic' for

Topic.rb

class Topic < ActiveRecord::Base
  attr_accessible :body, :forum_id, :title

  before_create :init_sort_column

  belongs_to :user
  belongs_to :forum
  validates :forum_id, :body, :title, presence: true

  has_many :comments, :as => :commentable

  default_scope order: 'topics.created_at DESC'

  private
  def init_sort_column
    self.active = self.created_at || Time.now
  end
end

Comment.rb

class Comment < ActiveRecord::Base
  attr_accessible :body, :commentable_id, :commentable_type, :user_id

  belongs_to :user
  belongs_to :commentable, :polymorphic => true

  before_create :update_parent_sort_column

  private

  def update_parent_sort_column
    self.topic.active = self.created_at if self.topic
  end

end

Answer 1

Didn't realise you were using a polymorphic association. Use the following:

def update_parent_sort_column
  commentable.active = created_at if commentable.is_a?(Topic)
  commentable.save!
end

Should do the trick.