class User
  include MongoMapper::Document
  many :comments
end

class Comment
  include MongoMapper::Document
  belongs_to :user
end

user = User.create!
user.comments.create!
user.destroy

class User
  include MongoMapper::Document
  many :comments
end

class Comment
  include MongoMapper::EmbeddedDocument
  belongs_to :user
end

user = User.create!
user.comments.create!
user.destroy