driver.rb

class Driver

  ....

  after_destroy :set_new_current_driver_to_insurance_detail
    
  protected
  def set_new_current_driver_to_insurance_detail
    @insurance_detail = self.insurance_detail
    @insurance_detail.update_attribute(:current_driver, @insurance_detail.drivers.first) if @insurance_detail.drivers.any? and @insurance_detail.current_driver_id == self.id
  end
  
end

driver_spec.rb

  describe "#set_new_current_driver_to_insurance_detail" do
    let(:driver) { Factory :driver }
    let(:insurance_detail) { Factory :insurance_detail }
    
    context "when insurance detail has no more drivers" do
      before(:each) do
        driver.update_attribute(:insurance_detail, insurance_detail)
        insurance_detail.update_attribute(:current_driver, driver)
      end
      it "should not change current_driver_id" do
        original_id = insurance_detail.current_driver_id
        driver.destroy
        insurance_detail.reload
        expect { insurance_detail.current_driver_id == original_id }
      end
    end
  end