fix for issue MongoEngine#904, also includes a test which checks the …

…error case I reported in the issue report.

mongoengine/base/document.py

@@ -686,7 +686,12 @@ def _from_son(cls, son, _auto_dereference=True, only_fields=None, created=False)
         # get the class name from the document, falling back to the given
         # class if unavailable
         class_name = son.get('_cls', cls._class_name)
-        data = dict(("%s" % key, value) for key, value in son.iteritems())
+        if PY3:
+            son_gen = son.items()
+        else:
+            son_gen = son.iteritems()
+
+        data = dict(("%s" % cls._db_field_map.get(key,key), value) for key, value in son_gen)
 
         # Return correct subclass for document type
         if class_name != cls._class_name:

tests/document/db_fields.py

@@ -0,0 +1,37 @@
+# -*- coding: utf-8 -*-
+import sys
+sys.path[0:0] = [""]
+import unittest
+from mongoengine import *
+
+__all__ = ("DbFieldParameterTest", )
+
+
+class DbFieldParameterTest(unittest.TestCase):
+
+    def test_document_db_field_validation(self):
+
+        # https://github.com/MongoEngine/mongoengine/issues/904
+
+        data = {'b': {'c': [{'x': 1.0, 'y': 2.0}]}}
+
+        class C(EmbeddedDocument):
+            x = FloatField(db_field='fx')
+            y = FloatField(db_field='fy')
+
+        class B(EmbeddedDocument):
+            c = ListField(EmbeddedDocumentField(C), db_field='fc')
+
+        class A(Document):
+            b = EmbeddedDocumentField(B, db_field='fb')
+
+        a = A(**data)
+
+        try:
+            a.validate()
+        except ValidationError as e:
+            self.fail("ValidationError raised: %s" % e.message)
+
+
+if __name__ == '__main__':
+    unittest.main()