Fixes .save _delta issue with DBRefs

Fixes hmarr/mongoengine#518

docs/changelog.rst

@@ -4,6 +4,8 @@ Changelog
 
 Changes in 0.6.X
 ================
+- Fixed .save() _delta issue with DbRefs
+- Fixed Django TestCase
 - Added cmp to Embedded Document
 - Added PULL reverse_delete_rule
 - Fixed CASCADE delete bug
@@ -317,4 +319,4 @@ Changes in v0.1.2
 
 Changes in v0.1.1
 =================
-- Documents may now use capped collections
+- 

mongoengine/base.py

@@ -1050,13 +1050,17 @@ Invalid data to create a `%s` instance.\n%s""".strip() % (cls._class_name, error
             for path in set_fields:
                 parts = path.split('.')
                 d = doc
+                dbref = False
                 for p in parts:
-                    if hasattr(d, '__getattr__'):
+                    if isinstance(d, DBRef):
+                        dbref = True
+                    elif hasattr(d, '__getattr__'):
                         d = getattr(p, d)
                     elif p.isdigit():
                         d = d[int(p)]
                     else:
                         d = d.get(p)
+                if not dbref:
                     set_data[path] = d
         else:
             set_data = doc

setup.cfg

@@ -4,9 +4,10 @@ test = nosetests
 [nosetests]
 verbosity = 2
 detailed-errors = 1
-with-coverage = 1
-#cover-html = 1
-#cover-html-dir = ../htmlcov
+#with-coverage = 1
+cover-html = 1
+cover-html-dir = ../htmlcov
 cover-package = mongoengine
 cover-erase = 1
 where = tests
+#tests = test_bugfix.py