class Post
  include Mongoid::Document
  has_many :comments
end

class Comment
  include Mongoid::Document
  belongs_to :post
end

1.9.3p392 :006 >   p = Post.create
 => #<Post _id: 51b984bbf52876e795000001, >
1.9.3p392 :007 > p.comments.create
 => #<Comment _id: 51b984c2f52876e795000002, post_id: "51b984bbf52876e795000001">
1.9.3p392 :008 > p.comments.new
 => #<Comment _id: 51b984c7f52876e795000003, post_id: "51b984bbf52876e795000001">
1.9.3p392 :009 > p.comments.last
 => #<Comment _id: 51b984c2f52876e795000002,post_id: "51b984bbf52876e795000001">

1.9.3p392 :011 > p.comments.entries
 => [#<Comment _id: 51b984c2f52876e795000002,post_id: "51b984bbf52876e795000001">, #<Comment _id: 51b984c7f52876e795000003, post_id: "51b984bbf52876e795000001">]
1.9.3p392 :012 > p.comments._loaded?
 => true
1.9.3p392 :013 > p.comments.last
 => #<Comment _id: 51b984c2f52876e795000002, post_id: "51b984bbf52876e795000001">