Can't wrap my head around appengine data store persistence

Question

Hi,

I've run into the "can't operate on multiple entity groups in a single transaction." problem when using APPENGINE FOR JAVA w/ JDO with the following code:

PersistenceManager pm = PMF.get().getPersistenceManager();

Query q = pm.newQuery("SELECT this FROM " + TypeA.class.getName() + " WHERE userId == userIdParam "); q.declareParameters("String userIdParam"); List poos = (List) q.execute(userIdParam);

for (TypeA a : allTypeAs) { a.setSomeField(someValue); } pm.close(); }

The problem it seems is that I can't operate on a multiple entities at the same time b/c they arent in the same entity group while in a transaction. Even though it doesn't seem like I'm in a transaction, appengine generates one because I have the following set in my jdoconfig.xml:

   <property name="datanucleus.appengine.autoCreateDatastoreTxns" value="true"/>

Fine. So far I think I understand.

BUT - if I replace TypeA in the above code, with TypeB - I don't get the error. I don't believe there is anything different between type a and type b - they both have the same key structure. They do have different fields but that shouldn't matter, right?

My question is - what could possible be different between TypeA and TypeB that they give this different behavior? And consequently what do you I fundamentally misunderstand that this behavior could even exist....

Thanks.

Answer 1

I haven't worked that much with App Engine, but from what I remember an entity group is defined as a hierarchy of objects with a root (tree). If your TypeB objects have a child property that's also a TypeB, it's possible that they're all in the same entity group. Likewise if they're all children to some other type.

If that's not the case you may need to detach the list of objects you get from the select, modify each one, and then create a new transaction for each one to save them one by one.
Take a look at updating using detach (the longer example toward the end of the sub-section).