issue-216.rb

require 'mongoid'

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

class User
  include Mongoid::Document
  has_many_related :memberships
  collection.remove
end

class Account
  include Mongoid::Document
  embeds_many :memberships
  collection.remove
end

class Membership
  include Mongoid::Document
  embedded_in :account, :inverse_of => :memberships
  validates_associated :user
  belongs_to_related :user
end

a = Account.create
a.memberships.build(:user => nil).valid?
Membership.new(:user => nil).valid?