Source in Meta.extra_kwargs do not add to fields - django

Question

I have few django models:
class User(AbstractBaseUser, PermissionsMixin):
email = models.EmailField(_('email address'), unique=True)
phone = models.CharField(
_('phone number'), max_length=16,
unique=True, blank=True, null=True,
default=None, validators=[phone_validator]
)
...
class UserExtension(models.Model):
user = models.OneToOneField(User, on_delete=models.CASCADE)
class Meta:
abstract = True
class Worker(UserExtension):
# some fields
class Administrator(UserExtension):
# some fields
View:
class AdministratorListView(ListAPIView):
permission_classes = (AllowAny,)
queryset = Administrator.objects.all()
serializer_class = serializers.AdministratorSerializer
And serializer:
class AdministratorSerializer(serializers.ModelSerializer):
class Meta:
model = Administrator
fields = ('network', 'email')
extra_kwargs = {'email': {'source': 'user.email'}}
But this is not working. Following traceback tell about field does not exist for my model:
Traceback (most recent call last):
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
response = get_response(request)
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/core/handlers/base.py", line 124, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
return view_func(*args, **kwargs)
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/views/generic/base.py", line 68, in view
return self.dispatch(request, *args, **kwargs)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/views.py", line 495, in dispatch
response = self.handle_exception(exc)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/views.py", line 455, in handle_exception
self.raise_uncaught_exception(exc)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/views.py", line 492, in dispatch
response = handler(request, *args, **kwargs)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/generics.py", line 201, in get
return self.list(request, *args, **kwargs)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/mixins.py", line 48, in list
return Response(serializer.data)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 765, in data
ret = super(ListSerializer, self).data
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 262, in data
self._data = self.to_representation(self.instance)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 683, in to_representation
self.child.to_representation(item) for item in iterable
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 683, in <listcomp>
self.child.to_representation(item) for item in iterable
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 510, in to_representation
fields = self._readable_fields
File "/home/vagrant/v_env/lib/python3.6/site-packages/django/utils/functional.py", line 37, in __get__
res = instance.__dict__[self.name] = self.func(instance)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 376, in _readable_fields
field for field in self.fields.values()
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 363, in fields
for key, value in self.get_fields().items():
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 1047, in get_fields
source, info, model, depth
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 1192, in build_field
return self.build_unknown_field(field_name, model_class)
File "/home/vagrant/v_env/lib/python3.6/site-packages/rest_framework/serializers.py", line 1304, in build_unknown_field
(field_name, model_class.__name__)
django.core.exceptions.ImproperlyConfigured: Field name `user.email` is not valid for model `Administrator`.
I looked at the source code and realized that the value from 'source' is passed to build_field as field_name argument and rest-framework simply cannot find this field ('user.email') in my model (Administrator)
part of get_fields method
build_field method