one_sided_child.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('testing')
end

class User
  include Mongoid::Document
  collection.remove
end

class Post
  include Mongoid::Document
  belongs_to_related :created_by, :class => "User"
  collection.remove
end

p user = User.create
p post = Post.create(:created_by => user)

one_sided_parent.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('testing')
end

class User
  include Mongoid::Document
  references_many :posts, :stored_as => :array
  collection.remove
end

class Post
  include Mongoid::Document
  collection.remove
end

p user = User.create
user.posts.create

p user.attributes

Whoops. The association on post should look like this:

class Post
  belongs_to_related :created_by, :class_name => "User"
end