使用Django Rest作為后端在做的項(xiàng)目中，Model是這樣的：

class Sample(models.Model):
    ...
    creater = models.ForeignKey(User, on_delete=models.CASCADE)
    ...

Serializers是這樣：

class SampleSerializer(serializers.ModelSerializer):
    class Meta:
        model = Sample
        fields = (...,'creater')

Views是這樣：

class SampleList(generics.ListCreateAPIView):
    queryset = Sample.objects.all()
    serializer_class = SampleSerializer
    permission_classes = (IsOwnerOrReadOnly,)

    def perform_create(self, serializer):
        ...
        serializer.save(creater=self.request.user,...)

問題是，我在執(zhí)行插入的時(shí)候，總是提示：

{"creater": ["This field is required."]}

百思不得其解，因?yàn)槊髅魍ㄟ^creater=self.request.user賦值了啊。
后來在Model中，將creater字段修改為

creater = models.ForeignKey(User, on_delete=models.CASCADE, null=True, blank=True)

問題解決。

但這是為什么呢？谷歌后發(fā)現(xiàn)：

The perform_create method is called by your View's create method after serializer validation. At this stage, DRF will have found and flagged the missing user field.

原來視圖中的perform_create操作晚于serializer的校驗(yàn)。上面的代碼中，perform_create前DRF已經(jīng)發(fā)現(xiàn)creater字段的缺失。

來看看rest_framework的源碼：

class CreateModelMixin(object):
    """
    Create a model instance.
    """
    def create(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        self.perform_create(serializer)
        headers = self.get_success_headers(serializer.data)
        return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)

    def perform_create(self, serializer):
        serializer.save()

    def get_success_headers(self, data):
        try:
            return {'Location': str(data[api_settings.URL_FIELD_NAME])}
        except (TypeError, KeyError):
            return {}

解決這個(gè)問題的方法有多種，除了上面的null=True, blank=True之外，還可以：

• 在serializer中設(shè)置這個(gè)字段readonly=True，或
• 在serializer中重載validate_user，或
• 使用DFR的currentuserdefault校驗(yàn)器。

參考地址。