gistfile1.rb

class User < ActiveRecord::Base
  def comments
    Post.where('comments.user_id' => self.id).all.select {|post|
      post.comments.select{|c| c.user_id == self.id}
    }.flatten.uniq
  end
end

class Post
    include Mongoid::Document

    embeds_many      :comments
end

class Comment
    include Mongoid::Document

    field  :user_id
    embedded_in :parent, :inverse_of => :comment
end

gistfile2.txt

I have several users, several posts and for every post, several comments.
Every post has one user. However the user table isn't on mongodb (I could talk with you about why. But please, that's not the question here).

I'm looking forward getting all the comments for one user.
So looping through all the posts to get the comments with user_id = x.

J'ai modifié pour que ca se rapporte plus à ton cas effectivement.