Temp fix.

mongo/mongo.go

@@ -102,6 +102,10 @@ func (c *MongoDataSourceCollection) Append(items ...interface{}) ([]db.Id, error
 			if items[i].(map[string]interface{})["_id"] == nil {
 				items[i].(map[string]interface{})["_id"] = id
 			}
+		case db.Item:
+			if items[i].(db.Item)["_id"] == nil {
+				items[i].(db.Item)["_id"] = id
+			}
 		}
 
 		args[i+1] = reflect.ValueOf(toInternal(items[i]))