Small explanation for Peno 3. How to use the concept of foreign keys in Google App engine and unidexed properties. How to pass objects from a servlet to a jsp .

1. Data Store
(Google App Engine)
José Luis Santos
joseluis.santos@cs.kuleuven.be

2. Servlet
DatastoreServicedatastore = DatastoreServiceFactory.getDatastoreService();
Entity person = new Entity("Person", "test");
person.setUnindexedProperty("name", "TestUser"); //Unindexed properties for these properties that you will
not filter on
Entity weddingPhoto = new Entity("Photo", person.getKey()); // We link this Entity to another (Foreign key)
weddingPhoto.setProperty("imageUrl", "http://scm-l3.technorati.com/11/02/05/25749/marriage.jpg");
weddingPhoto.setProperty("date", new Date());
Entity weddingVideo = new Entity("Video", person.getKey()); // We link this Entity to another (Foreign key)
weddingVideo.setProperty("videoUrl", "http://www.criticallayouts.com/images/rsgallery/original/just-
married-cans-ag1.gif");
weddingVideo.setUnindexedProperty("date", new Date());
datastore.put(person);
datastore.put(weddingPhoto);
datastore.put(weddingVideo);
req.setAttribute("Key", person.getKey()); //Sending an object to a jsp file
RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/store.jsp");
dispatcher.forward(req,resp);

3. JSP
DatastoreServicedatastore = DatastoreServiceFactory.getDatastoreService();
Key key = (Key)request.getAttribute("Key"); //We get an object not only an String
if (key != null){
Query userMediaQuery = new Query();
userMediaQuery.setAncestor(key);
// Ancestor queries return ancestors by default. This filter excludes the ancestor from query results.
userMediaQuery.addFilter(Entity.KEY_RESERVED_PROPERTY, Query.FilterOperator.GREATER_THAN, key);
// Returns both weddingPhoto and weddingVideo even though they are different entity kinds.
List<Entity> results = datastore.prepare(userMediaQuery).asList( FetchOptions.Builder.withDefaults());
for (inti=0;i<results.size();i++){
if (results.get(i).getKind().compareTo("Video")==0){%>
This is a photo:<br/><imgsrc="<%=results.get(i).getProperty("videoUrl")%>"></img><br/>
<% }else if (results.get(i).getKind().compareTo("Photo")==0){%>
This is a video:<br/><imgsrc="<%=results.get(i).getProperty("imageUrl")%>"></img><br/>
<% }%>