Question: I've got:

```
class Article
  include Mongoid::Document

  embeds_many :comments
end

class Comment
  include Mongoid::Document

  embedded_in :article
end

```

Since `Comment` is an embedded document in `Article`. How would I list down **ALL** Comments in order of `created_at`? 

Should I structure it so that Comment is not an embedded in `Article` or is there a way with the above schema?

Answer: 
If you're trying to query comments out of context of their articles, then clearly comment should be a top level entity and not embedded one.

In embedded case you can use map-reduce or aggregation framework. Regular query language won't help you here.
why not just `default_scope :order => 'created_at DESC'`? (or ASC).

You're going to want them to be in a consistent order across the site anyway.