Cross Model Validation

So I have this question:

Suppose I have these 3 models:

class Company < ActiveRecord::Base
  has_many :departments
  has_many :employees
end

class Department < ActiveRecord::Base
  has_many :employees
  belongs_to :company
end

class Employee < ActiveRecord::Base
  belongs_to :company
  belongs_to :department
end

Furthermore, suppose the domain I am modeling requires that I support
the existence of employees that do not belong to any department
(otherwise I would declare that Company has_many :employees, :through
=> :departments instead).

As I am working with the Department model, I am thinking that it is
conceivable that a mistake can be made in the code to allow an
employee from a company to be incorrectly assigned to a department in
another company. Hence, it might be a good idea to write some custom
validation to ensure an employee and a department belong to the same
company before they associated together.

Since associate a department and an employee together in one of two
ways, I would need validation logic in both models. So I am thinking
it would involve something like:

1. In the Department model

def validate
  self.errors.add(:employees, "must all belong to same company") if
self.employees.collect { |e| e.company_id }.uniq.length > 1
end

2 In the Employee model:

def validate
  self.errors.add(:department, "must belong to the same company") if
self.department && self.department != self.company
end

However, having to write two separate methods to support a single
business rule feels not so DRY or elegant. Is there a better way to
go?

Thanks!

Well one way to finesse this might be to have a fictional/notional
catch-all department for each company, like "company staff" which acts
as a placeholder for employes not actually belonging to a 'real'
department.