Question

I have two models, post and comment. Posts has many comments and comments belongs to posts.

What I’m trying to do, is to have a list of posts that is ordered by creation date, unless it has comments. Then it needs to take the creation date of the latest comment.

Now I know that I can include associations like this:

Post.find(:all, :include => [:comments], :order => "comments.created_at DESC, posts.created_at DESC")

But this will order all posts with comments first and then the posts without comments. I don’t want either ones ordered separately, but combined.

The solution also needs to be compatible with a paginate gem like will_paginate.

Edit

I have it now working with the following code:

posts_controller.rb

@posts = Post.order('updated_at DESC').page(params[:page]).per_page(10)

comments_controller.rb

@post = Post.find(params[:post_id])
@post.update_attributes!(:updated_at => Time.now)

Answer 1

I’d recommend:
Add a last_commented_at date field to your Post model. Whenever someone comments on the post update that field. It de-normalizes your db structure a bit, but your query will be faster and more strait-forward.