Fixes recursion error when resetting changed fields

Fixes #214  - thanks to wpjunior for the test case

mongoengine/document.py

@@ -162,15 +162,18 @@ class Document(BaseDocument):
         id_field = self._meta['id_field']
         self[id_field] = self._fields[id_field].to_python(object_id)
 
-        def reset_changed_fields(doc):
+        def reset_changed_fields(doc, inspected_docs=None):
             """Loop through and reset changed fields lists"""
+
+            inspected_docs = inspected_docs or []
+            inspected_docs.append(doc)
             if hasattr(doc, '_changed_fields'):
                 doc._changed_fields = []
 
             for field_name in doc._fields:
                 field = getattr(doc, field_name)
-                if hasattr(field, '_changed_fields') and field != doc:
-                    reset_changed_fields(field)
+                if field not in inspected_docs and hasattr(field, '_changed_fields'):
+                    reset_changed_fields(field, inspected_docs)
 
         reset_changed_fields(self)
         signals.post_save.send(self.__class__, document=self, created=created)

tests/document.py

@@ -1045,6 +1045,32 @@ class DocumentTest(unittest.TestCase):
         except ValidationError:
             self.fail()
 
+    def test_save_max_recursion_not_hit(self):
+
+        class Person(Document):
+            name = StringField()
+            parent = ReferenceField('self')
+            friend = ReferenceField('self')
+
+        Person.drop_collection()
+
+        p1 = Person(name="Wilson Jr")
+        p1.parent = None
+        p1.save()
+
+        p2 = Person(name="Wilson Jr2")
+        p2.parent = p1
+        p2.save()
+
+        p1.friend = p2
+        p1.save()
+
+        # Confirm can save and it resets the changed fields without hitting
+        # max recursion error
+        p0 = Person.objects.first()
+        p0.name = 'wpjunior'
+        p0.save()
+
     def test_update(self):
         """Ensure that an existing document is updated instead of be overwritten.
         """