更新Google App Engine的DataStore中的查询-Java

Question

I am newbie to app engine. I want to update some fields of my entity in datastore. For that i have created query like below but neither it updates my entity nor throws error. I don't know where i am going wrong.I referred this SO post but i have huge collection of data. Therefor, for me, its very hard to fetch hundreds of records and persist them.Please help me to solve this problem.

Code:

@ApiMethod(name = "updateUserProfile", httpMethod = HttpMethod.GET, path = "userfeedmasterendpoint/updateProfile")
public void updateUserProfile(@Named("userName") String uName,
            @Named("uAbout") String userAbout) 
    {
       EntityManager mgr = null;
       try {
                mgr = getEntityManager();
                Query query = mgr
                        .createQuery("update UserFeedMaster u set u.userAbout = :uAbout where u.userName=:userName");
                query.setParameter("userName", uName);
                query.setParameter("uAbout", userAbout);
                query.executeUpdate();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return;
       }

Answer 1

A query will only ask the datastore for a set of object satisfying a criteria and retrieve these. To update an object you need to retrieve it (with a query for instance), update this object and then put it back in the datastore (by calling your ORM put method).

Answer 2

If you are new to AppEngine and you want to use the datastore in an easy way, you should look at Objectify. It is a java ORM for the Datastore. Here is a short description of the project :

Objectify is a Java data access API specifically designed for the Google App Engine datastore. It occupies a "middle ground"; easier to use and more transparent than JDO or JPA, but significantly more convenient than the Low-Level API. Objectify is designed to make novices immediately productive yet also expose the full power of the GAE datastore.

https://code.google.com/p/objectify-appengine/