Django - counting foreign keys in models - django

Question

I know how I can count things with annotate (in my view) but I would like to do the same in model (so it would be more reusable).
For example (lets take an example from django documentation) I have this model:
class Author(models.Model):
name = models.CharField(max_length=100)
age = models.IntegerField()
class Publisher(models.Model):
name = models.CharField(max_length=300)
class Book(models.Model):
name = models.CharField(max_length=300)
pages = models.IntegerField()
price = models.DecimalField(max_digits=10, decimal_places=2)
rating = models.FloatField()
authors = models.ManyToManyField(Author)
publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)
pubdate = models.DateField()
class Store(models.Model):
name = models.CharField(max_length=300)
books = models.ManyToManyField(Book)
and I can use in view this:
from django.db.models import Count
pubs = Publisher.objects.annotate(num_books=Count('book'))
But how I do that in model?
I know this question is pretty basic (probably) but I'm pretty much beginner in django.
Thanks for answers!

Answer 1

You can use custom managers:
Django docs: Managers
class BookManager(models.Manager):
def get_queryset(self):
return super().get_queryset().annotate(num_books=Count('book'))
class Publisher(models.Model):
name = models.CharField(max_length=300)
books = BookManager()
Now you can call it like this:
pubs = Publisher.books.all()
And you will have num_books with your objects.

Answer 2

You can use classmethod for this.
class Publisher(models.Model):
...
#classmethod
def get_book_count(cls):
return cls.objects.annotate(num_books=Count('book'))
You can call this method as
pubs = Publisher.get_book_count()
Edit - Also check out the answer by #Navid2zp which might be a better solution for you.