How do I update a counter of an associated object before it is saved?

Question

I have an attribute on my Node object called cached_comment_count where I need it to be updated whenever a new comment is created.

This is part of my create action in my CommentsController :

@node = Node.find(params[:node_id])
@comment = current_user.comments.new(comment_params)
@comment.node = @node
@node.cached_comment_count = @node.comments.count

However, this doesn't work because when I do @node.comments.count that just returns the current comments on the @node object before this new @comment is saved.

What does work is if I do:

@node.cached_comment_count += 1

The obvious issue with this is that it is simply just an increment of 1 - and doesn't feel complete. The best, most complete, way is to get the comments.count after this save has taken place.

But I don't want to end up in an infinite loop.

What's the best way to approach this?

Answer 1

Just consider counter_cache rails functionality. You can check what is it by old, but still good Ryan Bates railscast

Answer 2

you may use counter_cache

With this declaration, Rails will keep the cache value up to date, and then return that value in response to the size method

 in comment.rb
      ##should look something like this
      ##add this column in node table....comments_count
       belongs_to :node, counter_cache: true

by using @node.comments.size

Answer 3

Why don't you add after create hook inside your Comment model.

class Comment < ActiveRecord::Base
  after_create :increment_comment_cache

  private

  def increment_comment_cache
    self.node.update cached_comment_count: self.node.comments.count
  end
end

Then remove your logic from controller, remove lines

@comment.node = @node
@node.cached_comment_count = @node.comments.count

This way, you get very DRY way of updating cache count, from any place you create comment it will automaticaly update count for you