text: from django.db.models.functions import Lag, Lead, NthValue, Ntile
  from django.test import SimpleTestCase
  
  
  class ValidationTests(SimpleTestCase):
      def test_nth_negative_nth_value(self):
          msg = 'NthValue requires a positive integer as for nth'
          with self.assertRaisesMessage(ValueError, msg):
              NthValue(expression='salary', nth=-1)
  
      def test_nth_null_expression(self):
          msg = 'NthValue requires a non-null source expression'
          with self.assertRaisesMessage(ValueError, msg):
              NthValue(expression=None)
  
      def test_lag_negative_offset(self):
          msg = 'Lag requires a positive integer for the offset'
          with self.assertRaisesMessage(ValueError, msg):
              Lag(expression='salary', offset=-1)
  
      def test_lead_negative_offset(self):
          msg = 'Lead requires a positive integer for the offset'
          with self.assertRaisesMessage(ValueError, msg):
              Lead(expression='salary', offset=-1)
  
      def test_null_source_lead(self):
          msg = 'Lead requires a non-null source expression'
          with self.assertRaisesMessage(ValueError, msg):
              Lead(expression=None)
  
      def test_null_source_lag(self):
          msg = 'Lag requires a non-null source expression'
          with self.assertRaisesMessage(ValueError, msg):
              Lag(expression=None)
  
      def test_negative_num_buckets_ntile(self):
          msg = 'num_buckets must be greater than 0'
          with self.assertRaisesMessage(ValueError, msg):
              Ntile(num_buckets=-1)
  