old_contents: from django.test import TestCase
  from .models import Rental
  
  
  class RentalTestCase(TestCase):
  
      def test_str(self):
          rental = Rental(name='Fitnessflow')
          self.assertEqual(rental.__str__(), 'Rental by Fitnessflow')
  
new_contents: import datetime
  from django.test import TestCase
  from django.core.exceptions import ValidationError
  from .models import Rental
  
  
  class RentalTestCase(TestCase):
  
      def test_clean__valid_rental(self):
          rental = Rental(
              start_date=datetime.datetime(2017, 3, 15),
              return_date=datetime.datetime(2017, 3, 17)
          )
          rental.clean()
  
      def test_clean__same_dates(self):
          rental = Rental(
              start_date=datetime.datetime(2017, 3, 15),
              return_date=datetime.datetime(2017, 3, 15)
          )
          rental.clean()
  
      def test_clean__invalid_rental(self):
          rental = Rental(
              start_date=datetime.datetime(2017, 3, 15),
              return_date=datetime.datetime(2017, 3, 13)
          )
          with self.assertRaises(ValidationError):
              rental.clean()
  
      def test_str(self):
          rental = Rental(name='Fitnessflow')
          self.assertEqual(rental.__str__(), 'Rental by Fitnessflow')
  
subject: Add test for constraint