diff --git a/rest_framework/mixins.py b/rest_framework/mixins.py
index d898ca128..4e8551f2f 100644
--- a/rest_framework/mixins.py
+++ b/rest_framework/mixins.py
@@ -12,6 +12,28 @@ from rest_framework.response import Response
 from rest_framework.request import clone_request
 
 
+def _get_validation_exclusions(obj, pk=None, slug_field=None):
+    """
+    Given a model instance, and an optional pk and slug field,
+    return the full list of all other field names on that model.
+
+    For use when performing full_clean on a model instance,
+    so we only clean the required fields.
+    """
+    include = []
+
+    if pk:
+        pk_field = obj._meta.pk
+        while pk_field.rel:
+            pk_field = pk_field.rel.to._meta.pk
+        include.append(pk_field.name)
+
+    if slug_field:
+        include.append(slug_field)
+
+    return [field.name for field in obj._meta.fields if field.name not in include]
+
+
 class CreateModelMixin(object):
     """
     Create a model instance.
@@ -117,18 +139,20 @@ class UpdateModelMixin(object):
         """
         # pk and/or slug attributes are implicit in the URL.
         pk = self.kwargs.get(self.pk_url_kwarg, None)
+        slug = self.kwargs.get(self.slug_url_kwarg, None)
+        slug_field = slug and self.get_slug_field() or None
+
         if pk:
             setattr(obj, 'pk', pk)
 
-        slug = self.kwargs.get(self.slug_url_kwarg, None)
         if slug:
-            slug_field = self.get_slug_field()
             setattr(obj, slug_field, slug)
 
         # Ensure we clean the attributes so that we don't eg return integer
         # pk using a string representation, as provided by the url conf kwarg.
         if hasattr(obj, 'full_clean'):
-            obj.full_clean()
+            exclude = _get_validation_exclusions(obj, pk, slug_field)
+            obj.full_clean(exclude)
 
 
 class DestroyModelMixin(object):
diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index b0372ab83..89b3d8ce9 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -158,6 +158,7 @@ class BaseSerializer(Field):
 
         # If 'fields' is specified, use those fields, in that order.
         if self.opts.fields:
+            assert isinstance(self.opts.fields, (list, tuple)), '`include` must be a list or tuple'
             new = SortedDict()
             for key in self.opts.fields:
                 new[key] = ret[key]
@@ -165,6 +166,7 @@ class BaseSerializer(Field):
 
         # Remove anything in 'exclude'
         if self.opts.exclude:
+            assert isinstance(self.opts.fields, (list, tuple)), '`exclude` must be a list or tuple'
             for key in self.opts.exclude:
                 ret.pop(key, None)
 
@@ -421,8 +423,8 @@ class ModelSerializer(Serializer):
         cls = self.opts.model
         opts = get_concrete_model(cls)._meta
         pk_field = opts.pk
-        while pk_field.rel:
-            pk_field = pk_field.rel.to._meta.pk
+        # while pk_field.rel:
+        #     pk_field = pk_field.rel.to._meta.pk
         fields = [pk_field]
         fields += [field for field in opts.fields if field.serialize]
         fields += [field for field in opts.many_to_many if field.serialize]
diff --git a/rest_framework/tests/generics.py b/rest_framework/tests/generics.py
index 2e7ce7279..2dcf01060 100644
--- a/rest_framework/tests/generics.py
+++ b/rest_framework/tests/generics.py
@@ -319,7 +319,7 @@ class TestCreateModelWithAutoNowAddField(TestCase):
         self.assertEquals(created.content, 'foobar')
 
 
-# Test for particularly ugly reression with m2m in browseable API
+# Test for particularly ugly regression with m2m in browseable API
 class ClassB(models.Model):
     name = models.CharField(max_length=255)
 
@@ -350,3 +350,35 @@ class TestM2MBrowseableAPI(TestCase):
         view = ExampleView().as_view()
         response = view(request).render()
         self.assertEquals(response.status_code, status.HTTP_200_OK)
+
+
+# Regression for #666
+
+class ValidationModel(models.Model):
+    blank_validated_field = models.CharField(max_length=255)
+
+
+class ValidationModelSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ValidationModel
+        fields = ('blank_validated_field',)
+        read_only_fields = ('blank_validated_field',)
+
+
+class UpdateValidationModel(generics.RetrieveUpdateDestroyAPIView):
+    model = ValidationModel
+    serializer_class = ValidationModelSerializer
+
+
+class TestPreSaveValidationExclusions(TestCase):
+    def test_pre_save_validation_exclusions(self):
+        """
+        Somewhat weird test case to ensure that we don't perform model
+        validation on read only fields.
+        """
+        obj = ValidationModel.objects.create(blank_validated_field='')
+        request = factory.put('/', json.dumps({}),
+                              content_type='application/json')
+        view = UpdateValidationModel().as_view()
+        response = view(request, pk=obj.pk).render()
+        self.assertEquals(response.status_code, status.HTTP_200_OK)