code: def validate_lms_user_id(self, value):
          
          enterprise_customer = self.context.get('enterprise_customer')
  
          try:
              # Ensure the given user is associated with the enterprise.
              return models.EnterpriseCustomerUser.objects.get(
                  user_id=value,
                  enterprise_customer=enterprise_customer
              )
          except models.EnterpriseCustomerUser.DoesNotExist:
              pass
  
          return None
docs: Validates the lms_user_id, if is given, to see if there is an existing EnterpriseCustomerUser for it.