Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

create a model with a OneToOneField

class SpecialUser(models.Model):
    special_user = models.OneToOneField(
    User,
    on_delete=models.CASCADE,
    primary_key=True,
    related_name="+")

Then create a mode serializer.

class SpecialUserSerializer(ModelSerializer):
    class Meta:
        model = SpecialUser
        fields = '__all__'

Now view the serializer

SpecialUserSerializer():
    special_user = ModelField(
        model_field=<django.db.models.fields.related.OneToOneField: special_user>,
        validators=[<UniqueValidator(queryset=SpecialUser.objects.all())>]
    )

Now try to serialize a new user:

s = SpecialUserSerializer(data={'special_user': 2})
s.is_valid()  # True
s.save()

ValueError: Cannot assign "2": "SpecialUser.special_user" must be a "User" instance.

for full stacktrace see pastebin

Expected behavior

I would expect that the model serializer would treat the one-to-one field same as a foreign key and create a PrimaryKeyRelatedField instead of the model field. If I do this explicitly then the serializer works as expected:

class SpecialUserSerializer(ModelSerializer):
    special_user = PrimaryKeyRelatedField(queryset=User.objects.all())
    class Meta:
        model = SpecialUser
        fields = '__all__'

Actual behavior

If you don't explicitly create the primary key related field in the serializer then you cannot use the primary key to serialize the object because Django complains that you must use a model not an integer, nor can you use a nested serialization of the one-to-one related model because then DRF complains that you must use an integer.

I would like to post a pull request to fix this myself, but it will take a little time. I don't think this is so urgent, since the work around is so easy. In fact, maybe I'm missing something totally obvious and this works as intended?