python - 'NoneType' object has no attribute 'display_name' in django

Question

I have an django project where there are some apps , and models.py for each app with some ForiegnKey Fields. like below. for example this is Master_data app and inside models :

class MaintenanceType(models.Model):
    name = models.CharField(max_length=10)
    display_name = models.CharField(max_length=100, null = True, blank = True)    
    def __str__(self):
        return self.display_name or self.name

calculator app model:

class FamilyGroup(models.Model):
    name = models.CharField(max_length=10, choices=name_choices)
    transaction = models.ForeignKey(Transaction, on_delete=models.CASCADE)
    family_type = models.ForeignKey(FamilySituation,
                                    on_delete=models.PROTECT,
                                    null=True,
                                    blank=True)
    maintenance_type = models.ForeignKey(
        MaintenanceType,
        on_delete=models.PROTECT,
        limit_choices_to={'name__in': ['Single', 'Couple']},
        null=True,
        blank=True)

so i have a property function in the FamilyGroup model :

@property
    def maintenance_type_rate(self):
        b = self.maintenance_type.display_name
        r = MaintenanceTypeRate.objects.filter(
            Q(rate__gte=0) & Q(maintenance_type__display_name=b))
        for d in r:
            return float(d.rate or 0)

im getting an error 'NoneType' object has no attribute 'display_name' the error occurs at the property function traceback:

File "/home/abdallaoss/cra-calculator/calculator/models.py", line 301, in maintenance_type_rate
    b = self.maintenance_type.display_name or

Exception Type: AttributeError at /calculator/transaction/
Exception Value: 'NoneType' object has no attribute 'display_name'

I tried this :

@property
    def maintenance_type_rate(self):
        try:
            b = self.maintenance_type.display_name
        except AttributeError:
            b = "No display_name found!"
        r = MaintenanceTypeRate.objects.filter(
            Q(rate__gte=0) & Q(maintenance_type__display_name=b))
        for d in r:
            return float(d.rate or 0)

question from:https://stackoverflow.com/questions/65858778/nonetype-object-has-no-attribute-display-name-in-django

Answer 1

Waitting for answers