DRF Serializer 的many参数是干嘛的

Posted 2021-09-13 ponponon

Django rest framework

什么时候需要用many参数呢？

• 当构造序列化的时候，传递的参数的QuerySet实例，就需要用many=True参数，即便QuerySet长度为0或者长度为1
• 当构造序列化的时候，传递的参数的某个模型实例（内置的User实例，或者自定义的模型实例），就需要用many=false参数

源码解析

class BaseSerializer(Field):
    def __init__(self, instance=None, data=empty, **kwargs):
        self.instance = instance
        if data is not empty:
            self.initial_data = data
        self.partial = kwargs.pop(\'partial\', False)
        self._context = kwargs.pop(\'context\', {})
        kwargs.pop(\'many\', None)
        super().__init__(**kwargs)
        
    def __new__(cls, *args, **kwargs):
        if kwargs.pop(\'many\', False):
            return cls.many_init(*args, **kwargs)
        return super().__new__(cls, *args, **kwargs)

    @classmethod
    def many_init(cls, *args, **kwargs):

        allow_empty = kwargs.pop(\'allow_empty\', None)
        child_serializer = cls(*args, **kwargs)
        list_kwargs = {
            \'child\': child_serializer,
        }
        if allow_empty is not None:
            list_kwargs[\'allow_empty\'] = allow_empty
        list_kwargs.update({
            key: value for key, value in kwargs.items()
            if key in LIST_SERIALIZER_KWARGS
        })
        meta = getattr(cls, \'Meta\', None)
        list_serializer_class = getattr(meta, \'list_serializer_class\', ListSerializer)
        return list_serializer_class(*args, **list_kwargs)

当使用如下的语句进行序列化操作的时候。
serializer = TweetSerializer(queryset, many=True)
queryset会被赋值给BaseSerializer对应的构造方法def __init__(self, instance=None, data=empty, **kwargs):中的instance参数。
当instance参数接受的是QuerySet实例，就需要加many=True，如果instance接受的某个模型实例，例如TweetModel

class TweetAPIView(mixins.ListModelMixin,
                   mixins.CreateModelMixin,
                   GenericAPIView
                   ):
    queryset = TweetModel.objects.all()
    serializer_class = TweetCreateSerializer

    def get(self, request: Request):
        return self.list(request)

    def list(self, request, *args, **kwargs):
        queryset = self.filter_queryset(self.get_queryset())

        serializer = TweetSerializer(queryset, many=True)
        return Response(serializer.data)

    def post(self, request: Request):
        return self.create(request)

class TweetModel(models.Model):
    user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)
    title = models.CharField(max_length=32, verbose_name=\'标题\')
    content = models.TextField(verbose_name=\'正文\')

    photos = models.ManyToManyField(Photo, blank=True)
    likes_count = models.BigIntegerField(default=0, null=True)
    comments_count = models.BigIntegerField(default=0, null=True)
    read_count = models.BigIntegerField(default=0, null=True)

    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        index_together = ((\'user\', \'created_at\'),)

    def __str__(self):
        # 这里是你执行 print(tweet instance) 的时候会显示的内容
        return \'{} {}: {}\'.format(self.created_at, self.user, self.content)