Merge remote-tracking branch 'origin/master'

docs/guide/gridfs.rst

@@ -46,7 +46,7 @@ slightly different manner.  First, a new file must be created by calling the
     marmot.photo.write('some_more_image_data')
     marmot.photo.close()
 
-    marmot.photo.save()
+    marmot.save()
 
 Deletion
 --------

mongoengine/base/document.py

@@ -332,7 +332,7 @@ class BaseDocument(object):
             pk = "None"
             if hasattr(self, 'pk'):
                 pk = self.pk
-            elif self._instance:
+            elif self._instance and hasattr(self._instance, 'pk'):
                 pk = self._instance.pk
             message = "ValidationError (%s:%s) " % (self._class_name, pk)
             raise ValidationError(message, errors=errors)