我有以下协会
class Employee
include Mongoid::Document
employee_id :name
references_many :companies, stored_as => :array, :inverse_of => :employees
end
class Company
include Mongoid::Document
field :name
references_many :employees, stored_as => :array, :inverse_of => :companies
end
现在如何检查employee_id
单个员工的唯一性company