This is a working example of a CRUD (create, read, update and delete) editor for Google App Engine and GXT 3.1. The editor supports Objectify Reference fields to other App Engine classes. If such a field is edited a sub editor appears which allows the user to select entities of the referenced class.
Furthermore the App Engine server part is designed to support paging, filtering and sorting of datasets.
A running instance of this Example can be tested at http://lunar-parsec-618.appspot.com/WebAdmin.html
Please be aware that this is just a small example that shows how such a feature could look like in your own project.