Maybe he forgot to precede it with the usual "if
(em.getTransaction().isActive())"?
David Fuelling wrote:
> Hi Max,
>
> I asked this question in the comments of your blog post, but I'll post
> it here as well for the "group" folks:
>
> Why is the transaction rolled-back at the end of the each
> keysOfBooksPublishedBetween() function call? Also, it is necessary to
> wrap the "get by key" query in a transaction, and if so why is that?
>
> Thanks!
>
> David
>
> On Oct 6, 6:00 pm, Max Ross <maxr+appeng...@google.com> wrote:
>> Welcome to Episode 4 of JDO/JPA Snippets That Work. Today we're going to
>> look at keys-only queries.
>>
>> Keys Only Queries
>>
>> If you use the low-level datastore api you may have noticed that the
>> com.google.appengine.api.datastore.Query class has a setKeysOnly() method.
>> If you call this method before you execute the query the datastore will
>> return com.google.appengine.api.datastore.Entity instances that have their
>> keys filled in but none of their properties. This can reduce consumption of
>> your Datastore Data Received from API quota, especially if you've got some
>> large entities, but, more importantly, it can also reduce consumption of
>> your Datastore CPU Time quota. How? Well, if the fulfillment of your query
>> requires an index or a merge join your query actually executes in two
>> stages: First it scans the index to find the keys of the entities that
>> match and then it issues additional scans to retrieve the entities uniquely
>> identified by the matching keys. If your query is keys-only we can skip
>> that second step entirely. That means faster queries!
>>
>> Now, JPA and JDO don't know anything about keys-only queries, but they do
>> give you the flexibility to either return your entire object or some subset
>> of the fields on your object. If you construct this subset to only contain
>> the primary key of your object, the App Engine implementation of JPA and JDO
>> will use a keys-only query. Let's look at some examples:
>>
>> JPA:
>> @Entity
>> public class Book {
>> @Id
>> @GeneratedValue(strategy=GenerationType.IDENTITY)
>> private Key id;
>>
>> private Date dateOfPublication;
>>
>> // getters and setters
>>
>> }
>>
>> Now let's implement a method that returns the Keys of all Books published
>> betweeen 2 years (we'll assume someone else is creating and closing an
>> EntityManager named 'em' for us):
>>
>> public List<Key> keysOfBooksPublishedBetween(EntityManager em, Date from,
>> Date to) {
>> em.getTransaction().begin();
>> try {
>> Query q = em.createQuery("select id from " + Book.class.getName()
>> + " where dateOfPublication >= :from AND dateOfPublication <=
>> :to");
>> q.setParameter("from", from);
>> q.setParameter("to", to);
>> return (List<Key>) q.getResultList();
>> } finally {
>> em.getTransaction().rollback();
>> }
>>
>> }
>>
>> JDO:
>>
>> @PersistenceCapable(identityType = IdentityType.APPLICATION, detachable =
>> "true")
>> public class Book {
>>
>> @PrimaryKey
>> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
>> private Key id;
>>
>> private Date dateOfPublication;
>>
>> // getters and setters
>>
>> }
>>
>> Now let's implement a method that returns the Keys of all Books published
>> betweeen 2 years (we'll assume someone else is creating and closing a
>> PersistenceManager named 'pm' for us):
>>
>> public List<Key> keysOfBooksPublishedBetween(PersistenceManager pm, Date
>> from, Date to) {
>> pm.currentTransaction().begin();
>> try {
>> Query q = pm.newQuery("select id from " + Book.class.getName()
>> + " where dateOfPublication >= :from && dateOfPublication <=
>> :to");
>> return (List<Key>) q.execute(from, to);
>> } finally {
>> pm.currentTransaction().rollback();
>> }}
>>
>> --------------------------------
>> Notice how we are only selecting the 'id' field from our Book class. This
>> is crucial. If you select any other fields your query will end up fetching
>> entire entities and the optimization will be lost.
>>
>> Max
> >
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