How to change a model after creating a ModelViewset? - django

Question

I am creating an API with django rest framework.
But I have a problem every time I want to modify a model when I have already created an associated ModelViewset.
For example, I have this model:
class Machine(models.Model):
name = models.CharField(_('Name'), max_length=150, unique=True)
provider = models.CharField(_("provider"),max_length=150)
build_date = models.DateField(_('build date'))
category = models.ForeignKey("machine.CategoryMachine",
related_name="machine_category",
verbose_name=_('category'),
on_delete=models.CASCADE
)
site = models.ForeignKey(
"client.Site",
verbose_name=_("site"),
related_name="machine_site",
on_delete=models.CASCADE
)
class Meta:
verbose_name = _("Machine")
verbose_name_plural = _("Machines")
def __str__(self):
return self.name
def get_absolute_url(self):
return reverse("Fridge_detail", kwargs={"pk": self.pk})
And this viewset:
class MachineViewSet(viewsets.ModelViewSet):
"""
A simple ViewSet for listing or retrieving machine.
"""
permission_classes = [permissions.IsAuthenticated]
serializer_class = MachineSerializer
queryset = cache.get_or_set(
'machine_machines_list',
Machine.objects.all(),
60*60*24*7
)
#swagger_auto_schema(responses={200: MachineSerializer})
def list(self, request):
serializer_context = {
'request': request,
}
queryset = cache.get_or_set(
'machine_machines_list',
Machine.objects.all(),
60*60*24*7
)
serializer = MachineSerializer(queryset, many=True, context=serializer_context)
return Response(serializer.data)
#swagger_auto_schema(responses={404: 'data not found', 200: MachineSerializer})
def retrieve(self, request, pk=None):
serializer_context = {
'request': request,
}
queryset = Machine.objects.all()
machine = get_object_or_404(queryset, pk=pk)
serializer = MachineSerializer(machine, context=serializer_context)
return Response(serializer.data)
If I want to add a field to my model, like for example a description. When I run the command python manage.py makemigrations
I get the error :
django.db.utils.ProgrammingError: column machine_machine.description does not exist
I have to comment my viewset to be able to run the makemigrations and migrate.
How can I avoid having to comment each time the viewsets when I want to modify a model?

Answer 1

In your view you have the following line in your class:
queryset = cache.get_or_set(
'machine_machines_list',
Machine.objects.all(),
60*60*24*7
)
Since this line is present in the class declaration and not in a method of the class it is executed when the class is created / interpreted. This causes it to fire a query, since a queryset will need to be evaluated (pickling Querysets will force evaluation) to cache it. The problem here is that your models haven't been migrated yet and your queries fail. You can write that code in the get_queryset method by overriding it if you want:
class MachineViewSet(viewsets.ModelViewSet):
"""
A simple ViewSet for listing or retrieving machine.
"""
permission_classes = [permissions.IsAuthenticated]
serializer_class = MachineSerializer
# Remove below lines
# queryset = cache.get_or_set(
# 'machine_machines_list',
# Machine.objects.all(),
# 60*60*24*7
# )
def get_queryset(self):
return cache.get_or_set(
'machine_machines_list',
Machine.objects.all(),
60*60*24*7
)
...
But this implementation is critically flawed! You are caching the queryset for a week. Lot's of new data can come in within that time period and you would be essentially serving stale data to your users. I would advise you to forego this sort of caching. Also you seem to be returning all the data from your view, consider some sort of filtering on this so that less memory is needed for such stuff.