Django Rest Framework: Disable field update after

Question

I'm trying to make my User model RESTful via Django Rest Framework API calls, so that I can create users as well as update their profiles.

However, as I go through a particular verification process with my users, I do not want the users to have the ability to update the username after their account is created. I attempted to use read_only_fields, but that seemed to disable that field in POST operations, so I was unable to specify a username when creating the user object.

How can I go about implementing this? Relevant code for the API as it exists now is below.

class UserSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = User
        fields = ('url', 'username', 'password', 'email')
        write_only_fields = ('password',)

    def restore_object(self, attrs, instance=None):
        user = super(UserSerializer, self).restore_object(attrs, instance)
        user.set_password(attrs['password'])
        return user


class UserViewSet(viewsets.ModelViewSet):
    """
    API endpoint that allows users to be viewed or edited.
    """
    serializer_class = UserSerializer
    model = User

    def get_permissions(self):
        if self.request.method == 'DELETE':
            return [IsAdminUser()]
        elif self.request.method == 'POST':
            return [AllowAny()]
        else:
            return [IsStaffOrTargetUser()]

Thanks!

Answer 1

UPDATE:

Turns out Rest Framework already comes equipped with this functionality. The correct way of having a "create-only" field is by using the CreateOnlyDefault() option.

I guess the only thing left to say is Read the Docs!!! http://www.django-rest-framework.org/api-guide/validators/#createonlydefault

Old Answer:

Looks I'm quite late to the party but here are my two cents anyway.

To me it doesn't make sense to have two different serializers just because you want to prevent a field from being updated. I had this exact same issue and the approach I used was to implement my own validate method in the Serializer class. In my case, the field I don't want updated is called owner. Here is the relevant code:

class BusinessSerializer(serializers.ModelSerializer):

    class Meta:
        model = Business
        pass

    def validate(self, data):
        instance = self.instance

        # this means it's an update
        # see also: http://www.django-rest-framework.org/api-guide/serializers/#accessing-the-initial-data-and-instance
        if instance is not None: 
            originalOwner = instance.owner

            # if 'dataOwner' is not None it means they're trying to update the owner field
            dataOwner = data.get('owner') 
            if dataOwner is not None and (originalOwner != dataOwner):
                raise ValidationError('Cannot update owner')
        return data
    pass
pass

And here is a unit test to validate it:

def test_owner_cant_be_updated(self):
    harry = User.objects.get(username='harry')
    jack = User.objects.get(username='jack')

    # create object
    serializer = BusinessSerializer(data={'name': 'My Company', 'owner': harry.id})
    self.assertTrue(serializer.is_valid())
    serializer.save()

    # retrieve object
    business = Business.objects.get(name='My Company')
    self.assertIsNotNone(business)

    # update object
    serializer = BusinessSerializer(business, data={'owner': jack.id}, partial=True)

    # this will be False! owners cannot be updated!
    self.assertFalse(serializer.is_valid())
    pass

I raise a ValidationError because I don't want to hide the fact that someone tried to perform an invalid operation. If you don't want to do this and you want to allow the operation to be completed without updating the field instead, do the following:

remove the line:

raise ValidationError('Cannot update owner')

and replace it with:

data.update({'owner': originalOwner})

Hope this helps!

Answer 2

My approach is to modify the perform_update method when using generics view classes. I remove the field when update is performed.

class UpdateView(generics.UpdateAPIView):
    ...
    def perform_update(self, serializer):
        #remove some field
        rem_field = serializer.validated_data.pop('some_field', None)
        serializer.save()

Answer 3

If you don't want to create another serializer, you may want to try customizing get_serializer_class() inside MyViewSet. This has been useful to me for simple projects.

# Your clean serializer
class MySerializer(serializers.ModelSerializer):
    class Meta:
        model = MyModel
        fields = '__all__'

# Your hardworking viewset
class MyViewSet(MyParentViewSet):
    serializer_class = MySerializer
    model = MyModel

    def get_serializer_class(self):
        serializer_class = self.serializer_class
        if self.request.method in ['PUT', 'PATCH']:
            # setting `exclude` while having `fields` raises an error
            # so set `read_only_fields` if request is PUT/PATCH
            setattr(serializer_class.Meta, 'read_only_fields', ('non_updatable_field',))
            # set serializer_class here instead if you have another serializer for finer control
        return serializer_class

setattr(object, name, value)

This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

Answer 4

This post mentions four different ways to achieve this goal.

This was the cleanest way I think: [collection must not be edited]

class DocumentSerializer(serializers.ModelSerializer):

    def update(self, instance, validated_data):
        if 'collection' in validated_data:
            raise serializers.ValidationError({
                'collection': 'You must not change this field.',
            })

        return super().update(instance, validated_data)

Answer 5

Another method would be to add a validation method, but throw a validation error if the instance already exists and the value has changed:

def validate_foo(self, value):                                     
    if self.instance and value != self.instance.foo:
        raise serializers.ValidationError("foo is immutable once set.")
    return value         

In my case, I wanted a foreign key to never be updated:

def validate_foo_id(self, value):                                     
    if self.instance and value.id != self.instance.foo_id:            
        raise serializers.ValidationError("foo_id is immutable once set.")
    return value         

See also: Level-field validation in django rest framework 3.1 - access to the old value