It looks like you're retrieving an existing object from the datastore,
adding a tag, and then re-persisting. Does this Set have any items in it
before you call add? Even if it's instantiated before you persist the
original entity, App Engine's JDO/JPA layer will return null for empty
lists, so this could be your issue. Just change your addNewTag method
slightly:
public void addNewTag(Key tagKey) {
if (selectedTags == null) {
selectedTags = new HashSet<Key>();
}
selectedTags.add(tagKey);
}
- Jason
On Sun, Nov 1, 2009 at 1:43 PM, dataStorm <[email protected]> wrote:
>
> Hello, can someone help me with this, I am trying to follow this post
> http://code.google.com/appengine/docs/java/datastore/relationships.html
> to
> create relationship between LINKS and TAGS.
>
> I am getting an error:
>
> Caused by:
>
> java.lang.NullPointerException
> at com.linkytty.Linkstore.addNewTag(Linkstore.java:101)
> at com.linkytty.TagLinker.doPost(TagLinker.java:52)
>
>
> This happens when I try to add a key using ".add()" :
>
> public void addNewTag(Key tagKey) {
>
> selectedTags.add(tagKey);
>
> }
>
>
> addNewTag() is called from my
>
> PersistenceManager pm = PMF.get().getPersistenceManager();
> Linkstore linkObject = pm.getObjectById(Linkstore.class, objectKey);
> linkObject.addNewTag(objectKey);
>
> I've logged my "objectKey" and etc. and it seems that it gets those
> right
> (
> String linkKeyId = req.getParameter("key");
> linkKeyID : aghsaW5reXR0eXIPCxIJTGlua3N0b3JlGAUM
>
> Key objectKey = KeyFactory.stringToKey(linkKeyId);
> objectKey: Linkstore(5)
> )
>
> My Classes:
>
> the class that creates links:
>
>
> public class Linkstore {
>
> @PrimaryKey
> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> private Key linkid;
>
> @Persistent(defaultFetchGroup = "true")
> private Set<Key> selectedTags;
>
>
> //STUFF HERE
>
> public Linkstore(User author, String linkcontent, String
> linkcomment,
> Date
> date) {
> this.author = author;
> this.linkcontent = linkcontent;
> this.linkcomment = linkcomment;
>
> this.date = date;
>
> selectedTags = new HashSet<Key>();
>
> }
>
> public Key getId() {
>
> return linkid;
>
> }
>
> public String getKeyAsString() {
> if (linkid == null)
> return null;
> return KeyFactory.keyToString(linkid);
> }
>
> // STUFF HERE
>
>
> public void addNewTag(Key tagKey) {
>
> selectedTags.add(tagKey);
>
> }
>
> }
>
> The class that captures the post:
>
>
> public void doPost(HttpServletRequest req, HttpServletResponse resp)
> throws IOException {
>
> String linkKeyId = req.getParameter("key");
> Key objectKey = KeyFactory.stringToKey(linkKeyId);
>
> String tagStringKey = req.getParameter("tagkey");
> Key tagObjectKey = KeyFactory.stringToKey(tagStringKey);
>
> PersistenceManager pm = PMF.get().getPersistenceManager();
> Linkstore linkObject = pm.getObjectById(Linkstore.class,
> objectKey);
>
> linkObject.addNewTag(objectKey);
>
> resp.sendRedirect("/");
> }
>
>
> ///
> Does the "@Persistent(defaultFetchGroup = "true")" make any
> difference?
>
>
> Help & Thanks!
>
> >
>
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