Django model with Charfield Count and Zero values

Question

I have a Django database model that has some attributes, one of them is a Charfield 'category' with Choices. I now want to annotate a queryset of this model with the count of the rows of each category. The thing is, the way i know to do it, only categories present in the queryset are getting counted, but i want a queryset with all categories annotated with the count (0 if no rows with this category).

This is what im doing at the moment:

Model.objects.all().values('category').annotate(total=Count('category'))

Is there a way to display all categories, including such with count 0?

Answer 1

You can not count categories that do not exist, since, well... these do not exist. The choices are not even transferred to the database.

Using a CharField with choices is not the ideal modeling for this. Typically it is better to make a model Category and then another MyModel with ForeignKey [Django-doc] to link a MyModel object to a Category , so:

class Category(models.Model):
     = models.CharField(max_length=128)

    def __str__(self):
        return self.name

class MyModel(models.Model):
    category = models.Category, on_delete=models.PROTECT

then we can create categories like:

Category.objects.bulk_create(
    Category(name='technology'),
    Category(name='art'),
    Category(name='science')
)

if we then link MyModel objects to these Category s, we can annotate the Category with the number of MyModel s with:

from django.db.models import Count

Category.objects.annotate(
    
)

Here the Category s that arise from this queryset will have an extra attribute .num_mymodel with the number of linked MyModel objects. Since a LEFT OUTER JOIN is performed, for Category s without any MyModel s, it will use 0 .