How to set 0, instead of null?-CodePudding

I have 'average_rating' field that takes values from field 'rates' and calculate average value. But when 'rates' field is empty, its shows null. views.py:

class BooksView(viewsets.ModelViewSet):
    serializer_class = BooksSerializer
    queryset = BooksModel.objects.all()
    filter_backends = [filters.DjangoFilterBackend, filtersrest.SearchFilter, filtersrest.OrderingFilter,]
    filterset_class = Myfilter
    filterset_fields = ('genres','date',)
    search_fields = ['title']
    def get_queryset(self):
        return BooksModel.objects.all().annotate(average_rating=models.ExpressionWrapper(Round(Avg('rates__rate'), 2), output_field=models.FloatField()), _sum_users_cart=Count('users__user'))

In the output i have this:

        "id": 18,
        "authors": [
            {
                "author": "Андрей курпатов"
            }
        ],
        "age": [],
        "tags": [],
        "users": [],
        "genres": [],
        "rates": [],
        "average_rating": null,
        "sum_users_cart": 0,
        "title": "Мозг и бизнес",
        "desc": "123",
        "url_photo": "https://s1.livelib.ru/boocover/1007572198/o/fde7/Andrej_Kurpatov__Mozg_i_biznes.jpeg",
        "is_published": true,
        "date": "Декабрь 29, 2022",
        "time": "20:20:02"

And i need something like this:

        "id": 18,
        "authors": [
            {
                "author": "Андрей курпатов"
            }
        ],
        "age": [],
        "tags": [],
        "users": [],
        "genres": [],
        "rates": [],
        "average_rating": 0,
        "sum_users_cart": 0,
        "title": "Мозг и бизнес",
        "desc": "123",
        "url_photo": "https://s1.livelib.ru/boocover/1007572198/o/fde7/Andrej_Kurpatov__Mozg_i_biznes.jpeg",
        "is_published": true,
        "date": "Декабрь 29, 2022",
        "time": "20:20:02"

CodePudding user response：

You may try to do it like this:

from django.db.models import Case, When, FloatField

return BooksModel.objects.all().annotate(
    average_rating=Case(
        When(rates__isnull=True, then=0),
        default=Round(Avg('rates__rate'), 2),
        output_field=FloatField()
    ), 
    _sum_users_cart=Count('users__user')
)

CodePudding user response：

you can add a default value for the average_ratingin your models.py

for example:

from django.db import models

class BooksModel(models.Model):
    ...
    average_rating = models.DecimalField(max_digits=12, decimal_places=2, default=0, null=False)
    ...

by doing that you ensure that whenever the value of average_rating is not set it will always be set to 0 instead of None.