code: def customers(self):
          '''
          List both any individuals signed up via the registration and payment system,
          and any individuals signed up without payment.
          '''
          return Customer.objects.filter(
              Q(privatelessoncustomer__lesson=self) |
              Q(registration__eventregistration__event=self)
          ).distinct(f customers(self):
          '''
          List both any individuals signed up via the registration and payment system,
          and any individuals signed up without payment.
          '''
          return Customer.objects.filter(
              Q(privatelessoncustomer__lesson=self) |
              Q(registration__eventregistration__event=self)
          ).distinct()
docs: List both any individuals signed up via the registration and payment system,
          and any individuals signed up without payment.