Add a sanity check to avoid running into unresolved related models.

rest_framework/models.py

-# Just to keep things like ./manage.py test happy
+import inspect
+
+from django.db import models
+
+
+def resolve_model(obj):
+    """
+    Resolve supplied `obj` to a Django model class.
+
+    `obj` must be a Django model class, or a string representation
+    of one.
+
+    String representations should have the format:
+        'appname.ModelName'
+    """
+    if type(obj) == str and len(obj.split('.')) == 2:
+        app_name, model_name = obj.split('.')
+        return models.get_model(app_name, model_name)
+    elif inspect.isclass(obj) and issubclass(obj, models.Model):
+        return obj
+    else:
+        raise ValueError("{0} is not a valid Django model".format(obj))

rest_framework/serializers.py

@@ -20,6 +20,7 @@ from django.db import models
 from django.forms import widgets
 from django.utils.datastructures import SortedDict
 from rest_framework.compat import get_concrete_model, six
+from rest_framework.models import resolve_model
 
 # Note: We do the following so that users of the framework can use this style:
 #
@@ -656,7 +657,7 @@ class ModelSerializer(Serializer):
             if model_field.rel:
                 to_many = isinstance(model_field,
                                      models.fields.related.ManyToManyField)
-                related_model = model_field.rel.to
+                related_model = resolve_model(model_field.rel.to)
 
                 if to_many and not model_field.rel.through._meta.auto_created:
                     has_through_model = True

rest_framework/tests/test_models.py

+from django.db import models
+from django.test import TestCase
+
+from rest_framework.models import resolve_model
+from rest_framework.tests.models import BasicModel
+
+
+class ResolveModelTests(TestCase):
+    """
+    `resolve_model` should return a Django model class given the
+    provided argument is a Django model class itself, or a properly
+    formatted string representation of one.
+    """
+    def test_resolve_django_model(self):
+        resolved_model = resolve_model(BasicModel)
+        self.assertEqual(resolved_model, BasicModel)
+
+    def test_resolve_string_representation(self):
+        resolved_model = resolve_model('tests.BasicModel')
+        self.assertEqual(resolved_model, BasicModel)
+
+    def test_resolve_non_django_model(self):
+        with self.assertRaises(ValueError):
+            resolve_model(TestCase)
+
+    def test_resolve_with_improper_string_representation(self):
+        with self.assertRaises(ValueError):
+            resolve_model('BasicModel')