获取关联的rails模型的索引

Question

I'm trying to set a default value for a rails model based on the number of siblings it has relative to an associated model. I've managed to get it working like this;

class Comment < ActiveRecord::Base

  belongs_to :post

  after_initialize :set_defaults  

  def set_defaults             
    comment_index = self.post.comments.count
    self.title ||= "Default title for comment #{comment_index}"  
  end
end 

But this is going to go back to the database and count the posts every time a new comment is saved and I'm trying to save on database calls. I was wondering if there is something already within rails that would perform this kind of functionality? I had a quick look through the API but it didn't turn up anything. Does anyone know?

Answer 1

The rails counter cache feature (you can learn about it here - http://railscasts.com/episodes/23-counter-cache-column ) can help with fetching the current comments count without counting them over and over again. But it won't save you the DB call to find the relevant post.

Also, note that both your current implementation and with counter cache you might end up with two or more comments having the same index and also inconsistent indexing (newer records having lower indexes than older ones), if you allow deletion of comments. If the latter is a problem, you can use the last comment's index to have more consistent indexing, or by caching the comments count in the parent post without decreasing the number when a comment is deleted.

Finally, if the comment index is for display only (like numbering the comments in the post page) you can do it on the frontend side. just render the comments by id or creation date and give each an index dynamically.

Hope it helps.