Django - How can you include annotated results in a serialized QuerySet?

Question

How can you include annotated results in a serialized QuerySet?

data = serializer.serialize(Books.objects.filter(publisher__id=id).annotate(num_books=Count(         


        

Answer 1

I did some research and found that serializer.serialize can only serialize queryset, and annotation just adds an attribute with each object of the queryset, so when you try to serialize a query, annotated fields aren't shown. This is my way of implementation:

from django.core.serializers.json import DjangoJSONEncoder

books = Books.objects.filter(publisher__id=id).annotate(num_books=Count('related_books')).values()
json_data = json.dumps(list(books), cls=DjangoJSONEncoder)

Answer 2

As shown in this post you can use SerializerMethodField in your Serializer:

class BooksSerializer(serializers.ModelSerializer):

  num_books = serializers.SerializerMethodField()

  def get_num_books(self, obj):
    try:
        return obj.num_books
    except:
        return None

It will serialize the annotated value (readonly)

Answer 3

To get count from specific columns, you must declare them via values method

>>>> Books.objects.filter(publisher__id=id).values('<group by field>').annotate(num_books=Count('related_books'))
[{'num_books': 1, '<group by field>': X}]

Answer 4

Based on the link, this has been solved by pull request (https://github.com/django/django/pull/1176) some months ago.

You need to add num_books as a property:

class Publisher():
    ....

    @property
    def num_books(self):
        return some_way_to_count('related_books')

and then call it like so:

data = serializer.serialize(Books.objects.filter(publisher__id=id)), use_natural_keys=True, extra=['num_books'])

I'm not too sure about the exact syntax, since I don't work much with serializers.