It lasts several days to find out the problem

# Why we should process a prefetched model(an instance)

I have nested to serializers named `Request` and `Invoice`. Also i have a primary key field named `staff` in Invoice model `staff = models.ForeignKey(User)`. Every attempts to set staff via API were failed. I were forced to see rest framework code and i found out that integer(in requested data) passed as an staff id converted to the User object(a ready and correct instance represents staff with the id). I think it is a good idea if we check it and if it's an instance and prefetched model avoid to `return self.get_queryset().get(pk=data)` it again! So i prepended an if clause to `to_internal_value` and the problem was resolved! 

## related problems
*. http://stackoverflow.com/questions/20681468/how-to-make-foreignkey-play-nice-with-none-in-drf
*. http://stackoverflow.com/questions/25874507/django-rest-framework-serializer-expecting-pk-getting-string
*. http://stackoverflow.com/questions/20681468/how-to-make-foreignkey-play-nice-with-none-in-drf

## my error message
```
{'staff': ['Incorrect type. Expected pk value, received User.']}
```

rest_framework/relations.py

@@ -7,7 +7,7 @@ from django.core.exceptions import ImproperlyConfigured, ObjectDoesNotExist
 from django.core.urlresolvers import (
     NoReverseMatch, Resolver404, get_script_prefix, resolve
 )
-from django.db.models import Manager
+from django.db.models import Manager, Model
 from django.db.models.query import QuerySet
 from django.utils import six
 from django.utils.encoding import smart_text
@@ -216,6 +216,8 @@ class PrimaryKeyRelatedField(RelatedField):
         return True
 
     def to_internal_value(self, data):
+        if isinstance(data, Model):
+            return data
         if self.pk_field is not None:
             data = self.pk_field.to_internal_value(data)
         try: