how to add annotate data in django-rest-framework queryset responses?

The accepted solution will hit the database as many times as results are returned. For each result, a count query to the database will be made.

The question is about adding annotations to the serializer, which is way more effective than doing a count query for each item in the response.

A solution for that:

models.py

class Author(models.Model):
    name = models.CharField(...)
    other_stuff = models...
    ...

class Book(models.Model):
    author = models.ForeignKey(Author)
    title = models.CharField(...)
    publication_year = models...
    ...

serializers.py

class BookSerializer(serializers.ModelSerializer):
    authors = serializers.IntegerField()

    class Meta:
        model = Book
        fields = ('id', 'title', 'authors')

views.py

class BookViewSet(viewsets.ModelViewSet):
    queryset = Book.objects.annotate(authors=Count('author'))
    serializer_class = BookSerializer
    ...

That will make the counting at database level, avoiding to hit database to retrieve authors count for each one of the returned Book items.

The queryset returned from get_queryset provides the list of things that will go through the serializer, which controls how the objects will be represented. Try adding an additional field in your Book serializer, like:

author_count = serializers.IntegerField(
    source='author_set.count', 
    read_only=True
)

Edit: As others have stated, this is not the most efficient way to add counts for cases where many results are returned, as it will hit the database for each instance. See the answer by @José for a more efficient solution.

Fiver's solution will hit the db for every instance in the queryset so if you have a large queryset, his solution will create a lot of queries.

I would override the to_representation of your Book serializer, it reuses the result from the annotation. It will look something like:

class BookSerializer(serializers.ModelSerializer):
     def to_representation(self, instance):
        return {'id': instance.pk, 'num_authors': instance.authors__count}

    class Meta:
        model = Book