Django rest PUT 请求序列化程序无法使用外键更新模型

我有两个模型，如下所示：

class Task(models.Model):
    user = models.ForeignKey(User, blank=True)
    what_task = models.CharField(max_length=100, )

    #This helps to print in admin interface
    def __str__(self):
        return u"%s" % (self.what_task)


class Step(models.Model):
    task = models.ForeignKey(Task, related_name='steps', on_delete=models.CASCADE, )
    what_step = models.CharField(max_length=50, blank=True, )

    #This helps to print in admin interface
    def __str__(self):
        return u"%s" % (self.what_step)

我已经编写了序列化程序：

class StepSerializer(serializers.ModelSerializer):
    class Meta:
        model = Step
        exclude = ('task',)

class TaskSerializer(serializers.ModelSerializer):
    steps = StepSerializer(many=True)
    class Meta:
        model = Task
        fields = '__all__'


    def create(self, validated_data):
        steps_data = validated_data.pop('steps')
        task = Task.objects.create(**validated_data)
        for step_data in steps_data:
            Step.objects.create(task=task, **step_data)
        return task 

    def update(self, instance, validated_data):
        steps_data = validated_data.pop('steps')

        instance.what_task = validated_data.get('what_task', instance.what_task)
        instance.how_often = validated_data.get('how_often', instance.how_often)
        instance.how_important = validated_data.get('how_important', instance.how_important)
        instance.why_perform = validated_data.get('why_perform', instance.why_perform)
        instance.why_important = validated_data.get('why_important', instance.why_important)
        instance.possible_improvement = validated_data.get('possible_improvement', instance.possible_improvement)
        instance.existing_solutions = validated_data.get('existing_solutions', instance.existing_solutions)
        instance.how_important_improvement = validated_data.get('how_important_improvement', instance.how_important_improvement)
        instance.advantages_of_improvement = validated_data.get('advantages_of_improvement', instance.advantages_of_improvement)

        instance.save()
        for step_data in steps_data:
            Step.objects.update(task=task, **step_data)
        return instance 

我的看法：

@api_view(['GET', 'POST'])
def task_list(request):
    """
    List all tasks, or create a new task.
    """
    if request.method == 'GET':
        print(request.user)
        tasks = Task.objects.filter(user=request.user.id)
        serializer = TaskSerializer(tasks, many=True)
        return Response(serializer.data)

    elif request.method == 'POST':
        serializer = TaskSerializer(data=request.data)
        print(request.data)
        if serializer.is_valid():
            serializer.save(user=request.user)
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        else:
            return Response(
                serializer.errors, status=status.HTTP_400_BAD_REQUEST)


@api_view(['GET', 'PUT', 'DELETE'])
def task_detail(request, id):
    """
    Get, udpate, or delete a specific task
    """
    try:
        task = Task.objects.get(id=id)
    except Task.DoesNotExist:
        return Response(status=status.HTTP_404_NOT_FOUND)

    if request.method == 'GET':
        serializer = TaskSerializer(task)
        return Response(serializer.data)

    elif request.method == 'PUT':
        serializer = TaskSerializer(task, data=request.data)
        if serializer.is_valid():
            serializer.save(user=request.user)
            return Response(serializer.data)
        else:
            return Response(
                serializer.errors, status=status.HTTP_400_BAD_REQUEST)

    elif request.method == 'DELETE':
        task.delete()
        return Response(status=status.HTTP_204_NO_CONTENT)

我的GET和POST请求工作正常（视图中的第一个函数），我的任务和相关步骤是在单个请求中创建的，但是PUT请求是错误的，我不确定我是否正确更新了步骤；如果我从serializerfor 循环中注释掉，则任务更新成功。

我应该修改什么以及如何修改以便我可以使用相关步骤更新特定任务？