getObjectsById on App Engine

Question

According to JDO, you can use PersistenceManager.getObjectsById to load multiple entity instances by their object id.

What kind of Collection does one need to use here? A Google Data Store Key does not work as object id.

Answer 1

Use PersistenceManager.newObjectIdInstance(), as such

List<Object> ids = new ArrayList<Object>();
for (Key key : keys) {
   ids.add(pm.newObjectIdInstance(Foo.class, key));
}

return (List<Foo>) pm.getObjectsById(ids);

I'm not sure however how expensive the call to newObjectIdInstance is (it shouldn't be from what I can see).

Answer 2

Not a direct answer, by as an alternative to getObjectsById, it seems that you can use a JDOQL query to load multiple entities by key:

public List getById(List keys) {
   Query q = pm.newQuery(
      "select from " + Book.class.getName() + " where :keys.contains(key)");
   return (List) q.execute(keys);
}

Apparently, this query is optimized to use an efficient low-level bulk API.

The order of the keys does get lost though, so you will have to re-sort the result in Java land.