RelatedObjectDoesNotExist with TabularInline and proxy models - django

Question

My app's models include the Service model along with three proxy models:
class Service(models.Model):
SERVICE_TYPE_CHOICES = [
('AR', 'Area'),
('OC', 'Occupancy'),
('CO', 'Consumption'),
]
service_type = models.CharField(max_length=2, choices=SERVICE_TYPE_CHOICES)
building = models.ForeignKey(Building, on_delete=models.CASCADE)
def __init__(self, *args, **kwargs):
super(HouseService, self).__init__(*args, **kwargs)
subclasses = {
'AR' : AreaService,
'OC' : OccupancyService,
'CO' : ConsumptionService,
}
self.__class__ = subclasses[self.service_type]
class AreaServiceManager(models.Manager):
def get_queryset(self):
return super(AreaServiceManager, self).get_queryset().filter(service_type='AR')
def create(self, **kwargs):
kwargs.update({'service_type': 'AR'})
return super(AreaServiceManager, self).create(**kwargs)
class AreaService(Service):
objects = AreaServiceManager()
def save(self, *args, **kwargs):
self.service_type = 'AR'
return super(AreaService, self).save(*args, **kwargs)
class Meta:
proxy = True
# Manager and Model class definitions omitted for the other two proxy models
This works as intended such that I can create instances for each proxy model transparently by registering them for the admin interface, and queries for e.g. AreaService.objects returns the proper subset of Service instances, and Service.objects.all() returns a list of instances not for the Service model, but for its subclasses.
But when I try to add a TabularInline to admin.py –
class ServiceInlineAdmin(admin.TabularInline):
model = Service
extra = 0
class BuildingAdmin(admin.ModelAdmin):
inlines = [ServiceInlineAdmin,]
– I get the following error message for the line self.__class__ = subclasses[self.service_type] in models.py (s. above):
__class__
<class 'services.models.Service'>
args
()
kwargs
{}
self
Error in formatting: RelatedObjectDoesNotExist: Service has no building.
subclasses
{'AR': <class 'services.models.AreaService'>,
'CO': <class 'services.models.ConsumptionService'>,
'OC': <class 'services.models.OccupancyService'>}
I retrieved all objects via the Django shell, and checked my database: All Service instances/DB entries are linked to a building. Commenting out the custom __init__ function solves the issue, and all linked services are displayed for each building (but I obviously lose the ability to work with the subclasses as required).
Any pointers to the cause of this error are greatly appreciated.

Answer 1

Problem solved. It turned out that the TabularInline form created 4 additional empty Service instances (regardless of extra = 0). To circumvent this issue, I modified my custom __init__ function like this:
def __init__(self, *args, **kwargs):
super(Service, self).__init__(*args, **kwargs)
subclasses = {
'AR' : AreaService,
'OC' : OccupancyService,
'CO' : ConsumptionService,
}
if self.id:
self.__class__ = subclasses[self.service_type]