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pythonjsondjangoannotateserialization
Nunovsky
Nunovsky

Reputation: 395

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

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!

Upvotes: 13

Views: 7565

Answers (4)

Martin Faucheux
Martin Faucheux

Reputation: 1005

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)

Upvotes: 10

ruddra
ruddra

Reputation: 51988

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)

Upvotes: 13

klasske
klasske

Reputation: 2184

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.

Upvotes: 1

xecgr
xecgr

Reputation: 5193

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}]

Upvotes: -1

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