handle None from model __str__; Fixes #753

AUTHORS

@@ -210,3 +210,4 @@ that much better:
  * Jay Shirley (https://github.com/jshirley)
  * DavidBord (https://github.com/DavidBord)
  * Axel Haustant (https://github.com/noirbizarre)
+ * Vyacheslav Murashkin (https://github.com/a4tunado)

mongoengine/base/document.py

@@ -229,7 +229,7 @@ class BaseDocument(object):
             u = self.__str__()
         except (UnicodeEncodeError, UnicodeDecodeError):
             u = '[Bad Unicode data]'
-        repr_type = type(u)
+        repr_type = str if u is None else type(u)
         return repr_type('<%s: %s>' % (self.__class__.__name__, u))
 
     def __str__(self):

tests/document/instance.py

@@ -103,6 +103,19 @@ class InstanceTest(unittest.TestCase):
 
         self.assertEqual('<Article: привет мир>', repr(doc))
 
+    def test_repr_none(self):
+        """Ensure None values handled correctly
+        """
+        class Article(Document):
+            title = StringField()
+
+            def __str__(self):
+                return None
+
+        doc = Article(title=u'привет мир')
+
+        self.assertEqual('<Article: None>', repr(doc))
+
     def test_queryset_resurrects_dropped_collection(self):
         self.Person.drop_collection()