has_one() in AR kind of weak?

Hi,

suppose I'm doing a rails system for the DMV.

The rules say:

A person may have 0 or 1 Drivers License.

So, I'd have these 2 tables:

class CreatePeople < ActiveRecord::Migration
  def self.up
    create_table :people do |t|
      t.column :name, :string
    end
  end

  def self.down
    drop_table :people
  end
end

class CreateDriverslicenses < ActiveRecord::Migration
  def self.up
    create_table :driverslicenses do |t|
      t.column :person_id, :integer
    end
  end

  def self.down
    drop_table :driverslicenses
  end
end

And these models:

class Person < ActiveRecord::Base
  has_one :driverslicense
end

class DriversLicense < ActiveRecord::Base
  belongs_to :person
end

What is the best way to enforce this rule:

A person may have 0 or 1 Drivers License.
??

I ran a simple test with console:

pete = Person.new

=> #<Person:0x2364d9c @attributes={"name"=>nil}, @new_record=true>

pete.name = 'Pete'

=> "Pete"

dl1 = Driverslicense.new

=> #<Driverslicense:0x235ac0c @attributes={"person_id"=>nil},
@new_record=true>

dl1.person_id = pete

=> #<Person:0x2364d9c @errors=#<ActiveRecord::Errors:0x23511ac
@errors={}, @base=#<Person:0x2364d9c ...>>,
@attributes={"name"=>"Pete", "id"=>1}, @new_record=false>

dl1.person.name

=> "Pete"

So, now Pete has a drivers license.

Let's try to give him another drivers license:

dl2 = Driverslicense.new

=> #<Driverslicense:0x233de2c @attributes={"person_id"=>nil},
@new_record=true>

dl2.person_id = pete

=> #<Person:0x2364d9c @errors=#<ActiveRecord::Errors:0x23511ac
@errors={}, @base=#<Person:0x2364d9c ...>>,
@attributes={"name"=>"Pete", "id"=>1}, @new_record=false>

dl2.person.name

=> "Pete"

It appears that dmv has assigned 2 drivers licenses to Pete.

Is it up to me to 'strengthen' the has_one() method in AR?

Here is info about my rails:

$ script/about
About your application's environment
Ruby version 1.8.4 (powerpc-darwin7.9.0)
RubyGems version 0.9.0
Rails version 1.1.6
Active Record version 1.14.4
Action Pack version 1.12.5
Action Web Service version 1.1.6
Action Mailer version 1.2.5
Active Support version 1.3.1

-Pete
http://GoodJobFastCar.com
goodjobfastcar@gmail.com

You could add a uniqueness validation on the Driverslicense model

class Driverslicense < ActiveRecord::Base

validates_uniqueness_of :person_id

end

-Jonathan.