code: public K getSavedId() {
          if (dbObjects.length == 0) {
              throw new MongoException("No objects to return");
          }
          if (dbObjects[0] instanceof JacksonDBObject) {
              throw new UnsupportedOperationException("Generated _id retrieval not supported when using stream serialization");
          }
          return jacksonDBCollection.convertFromDbId(dbObjects[0].get("_id"));
      }
docs: Get the saved ID.  This may be useful for finding out the ID that was generated by MongoDB if no ID was supplied.
  
  @return The saved ID
  @throws MongoException If no objects were saved