Create say a UserProfile class which has a One2One relation to the User class, where the id is an integer.

class UserProfile():
    user = models.OneToOneField(User, on_delete=models.CASCADE, primary_key=True)

If you define the view set with lookup_field='user_id' then the generated swagger doc defaults the lookup_field as a string instead of integer
class UserProfileViewSet(viewsets.GenericViewSet):
queryset = UserProfile.objects.all()
serializer_class = UserProfileSerializer
lookup_field = 'user_id'

I'm currently working on a PR for this: #482