Override serializer.data in Django REST Framework

Question

I've been trying to alter the value of a form field the Django REST Framework's admin panel and for some reason the change never takes place. I have the serializer below

class SomeView(ModelViewSet):
  queryset = MyModel.objects.all()
  serializer_class = MyModelSerializer

  # I Want to override this and change the POST data
  def perform_create(self, serializer):
     user = self.request.user.id

     # this was a form field where I manually entered the user ID
     # I now want it to default to the logged in user
     serializer.data['user'] = user

     # This returns the original user that was entered into the form field
     print serializer.data

I checked out serializer.data with dir() and it's just a python dictionary so I can't figure out why I can't modify the value. As a test, I tried to add extra values but that doesn't work either

# this doesnt work
serializer.data['some_new_field'] = 'test'

EDIT

On another note, I can copy the data and edit it

fake_data = serializer.data.copy()
fake_data['old_value'] = 'new value'

However, it always fails to validate

serializer = MyModelSerializer(data=fake_data)
serializer.is_valid() # returns false

EDIT EDIT:

Ok, so the validation error was caused by Django returning a SimpleLazyObject. Everything works now when I perform a copy of the data, but I'm really curious as to why I can't edit serializer.data directly without copying it. The problem is solved now, but if anyone can provide insight on the issue just for curiosity, that would be awesome.

Answer 1

I checked out serializer.data with dir() and it's just a python dictionary so I can't figure out why I can't modify the value.

While the value returned from Serializer.data is indeed a dictionary, Serializer.data is not a simple instance variable.

If you look at rest_framework/serializers.py:

class Serializer(BaseSerializer, metaclass=SerializerMetaclass):
    # [...]
    @property
    def data(self):
        ret = super().data
        return ReturnDict(ret, serializer=self)

ReturnDict inherits from OrderedDict, but you still get a new dictionary every time you access Serializer.data.

The real data is in _data, however as noted by the underscore you might not want to modify that either as it is not intended to be public. The values are filled by Serializer.to_representation() which you could override on the viewset.

As for the second part: ModelViewSet defines get_serializer() that is called with the request POST data to create the serializer you want to modify. I'd suggest try to change the input data before the serializer is created, instead.