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('related_books')), use_natural_keys=True)

However the key/value pare {'num_books': number} is not include into the json result.

I've been searching for similar questions on the internet, but i didn't found a solution that worked for me.

Here is a similar case: http://python.6.x6.nabble.com/How-can-you-include-annotated-results-in-a-serialized-QuerySet-td67238.html

Thanks!

score 13 · Answer 1 · answered Aug 21 '14 at 07:13

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)

score 9 · Answer 2 · answered Mar 31 '20 at 08:14

9

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)

answered Mar 31 '20 at 08:14

Martin Faucheux

884
9
26

3

this should be the accepted answer it works perfectly! – Duilio Nov 13 '20 at 17:49
1

This is the correct answer. AIt is also possible to do the same annotation logic in the `serializers.SerializerMethodField()` method, however not preferable if already done on `queryset` to avoid duplicating code. – Gevorg Hakobyan Apr 17 '21 at 21:19
1

There is no ModelSerializer class in Django. – bparker Apr 13 '22 at 21:36
I meant drf's ModelSerializer https://www.django-rest-framework.org/api-guide/serializers/#modelserializer – Martin Faucheux Apr 14 '22 at 13:08

score 1 · Answer 3 · answered Aug 20 '14 at 09:39

1

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.

answered Aug 20 '14 at 09:39

klasske

2,164
1
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Thanks for the help, but i can't make it work. Also, if my Model is the Django built in User (`from django.contrib.auth.models import User`) how do you do it? – Nunovsky Aug 20 '14 at 16:41
1

As of October 2017, the pull request has been closed without being merged, and the ticket is still open. – seb Oct 20 '17 at 08:03

score -1 · Answer 4 · answered Aug 20 '14 at 09:33

-1

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}]

answered Aug 20 '14 at 09:33

xecgr

5,095
3
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The count that i make is correct, the problem is the serializer class. But thanks for the help! – Nunovsky Aug 20 '14 at 16:42

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

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