mongoid uniqueness validation many-to-many relation

Question

I have following association

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

Now How can I check the uniqueness of employee_id of employee within a single company