Save a many-to-many model in Django/REST?

I'm writing a REST API for my Django app, and can't get POST requests to work on one model. Here's the model in question:

class ProjectNode(models.Model):
    name = models.CharField(max_length=60)
    place = models.CharField(max_length=150)
    time_spent = models.BigIntegerField()
    parent_project = models.ForeignKey(Project, related_name='tasks')
    workers = models.ManyToManyField(User, related_name='tasks_can_do')

    def __str__(self):
        return self.name

The User model just holds a name field at the moment. Here's my serializer for ProjectNode:

class ProjectNodeSerializer(serializers.ModelSerializer):
    class Meta:
        model = ProjectNode
        fields = ('id', 'name', 'place', 'time_spent', 'workers',)

And here's the API view (from views.py):

class WebProjectNodeListView(generics.ListCreateAPIView):
    queryset = ProjectNode.objects.all()
    serializer_class = ProjectNodeSerializer

    def pre_save(self, obj):
        obj.parent_project = Project.objects.get(pk=self.request.DATA['parent_project'])
        for worker_pk in self.request.DATA['workers']:
            obj.workers.add(User.objects.get(pk=worker_pk))
        obj.final_worker = User.objects.get(pk=self.request.DATA['final_workers'])

I tried a simpler version yesterday at first, which only had the Project ForeignKey relationship, and it seemed to work, so I thought that using add would work too, but I get an error when testing out the API with httpie (I already added some users and projects, and am sure I get their id's correctly). Here's the request:

http POST :8000/api/tasks/ name="newtask" place="home" time_spent:=50 parent_project:=1 workers:=[1]

And I get this error:

"<ProjectNode: newtask>" needs to have a value for field "projectnode" before this many-to-many relationship can be used.

And the traceback also points to this line of code:

obj.workers.add(User.objects.get(id=worker_pk))

Now, I get the feeling that this is because I'm trying to update the relationship on the User object before a ProjectNode object is created in the database, but I'm not sure how to resolve this?

DRF doesn't works create models which are nested serializers objects or Many to Many fields. So is necessary to override Serializer create method and create/get M2M models before create ProjectNode. Try to override create(self, validated_data) in your serializer and work with your data inside this method..

Example:

My model Project has M2M relation with ProjectImages. In ProjectSerializer I override create method like this.

 def create(self, validated_data):
        try:
            # Remove nested and M2m relationships from validated_data
            images = validated_data.pop('projectimage_set') if 'projectimage_set' in validated_data else []

            # Create project model
            instance = Project(**validated_data)
            if status:
                instance.set_status(status)

            project = instance.save()

            # Create relations
            for image in images:
                ProjectImage.objects.create(project=project, **image)

        except exceptions.ValidationError as e:
            errors_messages = e.error_dict if hasattr(e, 'error_dict') else e.error_list
            raise serializers.ValidationError(errors_messages)

        return project

Hope this help!