Django difficulty in displaying the data(count)

Question

Im new to django and trying to learn with building a forum

my model

class Subject(models.Model):
    name=models.CharField(max_length=128)
    created=models.DateTimeField('Created Date')


    def __str__(self):
        return self.name

class Book(models.Model):
    book_subject=models.ForeignKey(Subject,on_delete=models.CASCADE)
    book_title=models.CharField(max_length=128)
    url=models.URLField()
    votes=models.IntegerField(default=0)


    def __str__(self):
        return self.book_title

my query to database in django shell

Subject.objects.all()
>>>[<Subject: electronics>, <Subject: compter science>]

q=Subject.objects.get(id=2)
q.book_set.all()
>>>[<Book: python django>, <Book: python flask>]

How should i get the number of books for each subjects. (to get count of books for electronics and computer science subjects) I know this could be straight forward answer. I wanted to show the template as in forum where it displays subject name and number of books it contains

Can someone help me with query to get number of books for each subjects

Any help is much appreciated..Thanks in advance

Answer 1

If you only need the number of books for a certain subject, there is count

Subject.objects.get(id=2).book_set.count()

If you need the subjects with a count for the number of books for them you can annotate

from django.db.models import Count
subjects = Subject.objects.annotate(num_books=Count('book'))

for subj in subjects:
    print subj.num_books