persona.rb

  validates_uniqueness_of :dni, :allow_nil => true, :allow_blank=>true, :message=>I18n.t('persona.dni_already_exists')
  validates_uniqueness_of :dni, :scope=>[:nombres, :apellidos], :allow_nil => true, :message=>I18n.t('persona.person_already_exists')

persona_test.rb

  should "demand DNI to be unique" do
    Persona.make(:dni => "1")
    assert_raise(ActiveRecord::RecordInvalid) { Persona.make(:dni => "1") }
    assert_nothing_raised(ActiveRecord::RecordInvalid) { Persona.make(:dni => "2") }
  end

#  this helper won't work with :allow_nil	
#  should_require_unique_attributes :dni
	
	should "allow no repetition of nombres, apellidos unless DNI is specified" do
	  Persona.make(:nombres => "Juan", :apellidos => "Topo")
		assert_raise(ActiveRecord::RecordInvalid) do
			p = Persona.make_unsaved(:nombres => "Juan", :apellidos => "Topo")
			p.save!
		end
		assert_nothing_raise(ActiveRecord::RecordInvalid) do
			p = Persona.make_unsaved(:dni => "5", :nombres => "Juan", :apellidos => "Topo")
			p.save!
		end
	end