Question

Short question.
I have two models:

class Author(models.Model):
    name = models.CharField(max_length=250)

class Book(models.Model):
    title = models.CharField(max_length=250)
    author = models.ManyToManyField(Author)

One view:

def filter_books(request):
    book_list = Book.objects.filter(...)

How can I display in template next content:

Authors in selected books:
    Author1: book_count
    Author2: book_count
    ...

Answer 1

Let’s build the query step by step.

First, get the authors who have a book in book_list.

authors = Author.objects.filter(book__in=book_list)

The trick is to realise that an author will appear once for each book in book_list. We can then use annotate to count the number of times the author appears.

# remember to import Count!
from django.db.models import Count

authors = Author.objects.filter(book__in=book_list
          ).annotate(num_books=Count('id')

In the template, you can then do:

Authors in selected books:
{% for author in authors %}
{{ author.name }}: {{ author.num_books }}<br />
{% endfor %}