Fixed EmbeddedDocuments with id also storing _id (#402)

mongoengine/base/document.py

@@ -262,6 +262,8 @@ class BaseDocument(object):
                 data[field.db_field] = value
 
         # If "_id" has not been set, then try and set it
+        Document = _import_class("Document")
+        if isinstance(self, Document):
             if data["_id"] is None:
                 data["_id"] = self._data.get("id", None)
 

tests/document/instance.py

@@ -444,6 +444,13 @@ class InstanceTest(unittest.TestCase):
         self.assertEqual(Employee(name="Bob", age=35, salary=0).to_mongo().keys(),
                          ['_cls', 'name', 'age', 'salary'])
 
+    def test_embedded_document_to_mongo_id(self):
+        class SubDoc(EmbeddedDocument):
+            id = StringField(required=True)
+
+        sub_doc = SubDoc(id="abc")
+        self.assertEqual(sub_doc.to_mongo().keys(), ['id'])
+
     def test_embedded_document(self):
         """Ensure that embedded documents are set up correctly.
         """