如何将Django中average_rating的null值替换为0？

我有一个average_rating字段，通过rates字段取值计算平均值，但当rates字段为空时，该字段显示null。

我的views.py代码如下：

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'))

当前输出示例：

"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"

期望输出将average_rating改为0：

"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"

方法一：数据库查询阶段处理（推荐） #

使用Django的Coalesce函数，该函数返回传入参数中的第一个非空值，可直接将Avg的空结果替换为0。

1. 先导入Coalesce：

from django.db.models import Coalesce

2. 修改get_queryset中的annotate逻辑：

def get_queryset(self):
    return BooksModel.objects.all().annotate(
        average_rating=models.ExpressionWrapper(
            Round(Coalesce(Avg('rates__rate'), 0), 2), 
            output_field=models.FloatField()
        ), 
        _sum_users_cart=Count('users__user')
    )

这样数据库会直接返回0而非null，无需后续处理。

方法二：序列化器阶段处理 #

若不想修改查询逻辑，可在BooksSerializer中对字段值进行替换：

方式1：设置默认值 #

class BooksSerializer(serializers.ModelSerializer):
    average_rating = serializers.FloatField(default=0)

    class Meta:
        model = BooksModel
        fields = '__all__'  # 或指定包含average_rating的字段列表

方式2：使用SerializerMethodField自定义取值逻辑 #

class BooksSerializer(serializers.ModelSerializer):
    average_rating = serializers.SerializerMethodField()

    def get_average_rating(self, obj):
        return obj.average_rating if obj.average_rating is not None else 0

    class Meta:
        model = BooksModel
        fields = '__all__'

此方法在序列化时将null替换为0，适合无需数据库层面处理的场景。

内容的提问来源于stack exchange，提问作者kuanan