python - Django Rest Framework SerializerMethodField 类型错误 : "' RelatedManager' object does not support indexing"

Question

我在序列化器上有两个 SerializerMethodFields。其中之一返回 obj.somelist.count() 正常。但是在我要求其他方法返回 obj.somelist[0] 的地方，我得到了一个 TypeError 'RelatedManager' object does not support indexing。非常感谢任何建议。这是一些代码:

class TripPlaceSerializer(serializers.ModelSerializer):

    class Meta:
        model = TripPlace
        fields = ('trip', 'place', )


class StopSerializer(serializers.ModelSerializer):

    class Meta:
        model = TripPlace
        fields = ('place', )
        depth = 1


class TripSerializer(serializers.ModelSerializer):
    stops = StopSerializer(read_only=True, many=True)
    stops_count = serializers.SerializerMethodField()
    car = CarSerializer(read_only=True, many=False)
    origin = serializers.SerializerMethodField()
    # final = serializers.SerializerMethodField()

    class Meta:
        model = Trip
        fields = ('id', 'name', 'owner', 'car', 'stops_count', 'stops', 'origin', )

    def get_stops_count(self, obj):
        return obj.stops.count()

    def get_origin(self, obj):
        return obj.stops.first()

Answer 1

obj.somelist 是 RelatedManager，无论底层关系是什么，您都无法直接对其进行索引。您应该调用 RelatedManager 的 first 方法来获取第一个对象:

def get_origin(self, obj):
    return obj.somelist.first()

或者您可以使用不同于 0 的不同索引来索引(或切片)管理器的查询集:

def get_origin(self, obj):
    return obj.somelist.all()[ind]