Want to sort by association records count in Datamapper

Question

Lets say I have the following DataMapper resources:

class Post
  include DataMapper::Resource 

  has n, :comments
  ...

end  

class Comment
  include DataMapper::Resource 

  belongs_to :post
  ...

end

To get the ordered list of posts, I know you can do:

@posts = Posts.all(:order => :date.desc)

But lets say I want to display all the Posts ordered descending by how many comments they have. How would I do that?

Answer 1

A good way to do this for performance reasons is to cache the comment_count value for the post, which would then give you an attribute to use for the :order, maybe like :order => :comment_count_cache.desc . This is easily setup by adding an after creation hook for the Comment model.

Answer 2

You can alternatively use sort_by, which calls a separate query:

@post = Post.all(:order => :date.desc).sort_by { |post| -post.comments.count }

If you want to change the order, you can take out the minus sign:

@post = Post.all(:order => :date.desc).sort_by { |post| post.comments.count }

This is nice syntactically, but like adamaig points out, it'll probably perform worse than caching the number of comments in the db because you are adding an additional SQL query.

Answer 3

I believe that the SQL you want to generate is:

SELECT posts.*, COUNT(comments.id) AS comments_count FROM posts
JOIN comments ON posts.id = comments.post_id
GROUP BY posts.id ORDER BY comments_count DESC

As far as I know, this isn't something you can do programmatically with the Query class. Just drop into SQL for this.