I have a list of POJOs which have some uninitialized collection properties (one-to-many). Is there a way to fetch this collection property for each of the POJOs in a single query, without one query per POJO in a loop?

pojo1.children
pojo2.children
pojo3.children
pojo4.children
...

Actually the problem goes one step further: The direct collection property is actually initialized but the objects in this collection again have set properties which I need to fetch now, without a query for every one of them... I can only think of doing it manually. Collect the ids of the needed objects in a loop, then execute a query with an in clause. But if I do this I have a separate list of the late-fetched objects and have to somehow distribute/associate them with original list of POJOs.

Hibernate version: 3.0.5
Name and version of the database you are using: Oracle 9

If your session is still open, just accessing the property will cause hibernate to initialize it. Assuming that your session will be closed prior to accessing these collections, you have use Hibernate.initialize(pojo.children) to force initialization of the collection prior to closing the session.

You should also look at the batch-size property in the reference docs.

I know that the initialization is occurring transparently. But a loop over the pojos, calling initialize on each one, will result in n database hits, right? That's what I must avoid.

as pksiv said, look at the batch-size attribute.

Both of you are completely right (credits allotted), batch-size did the trick. Thanks a lot.

I could not imagine that this would work because previously I made the experience that pojos that I had loaded via a criteria query were reloaded when I wanted to access them via session.load() later on - although the query did return them already and Hibernate knew about them. From this I concluded that not even criteria queries, based on a single entity, would be cached (without explicit query cache and L2 level cache). But if they are not cached how could "batch-size" know about other pojos which need initialization? This remains in the dark for me... but somehow it works ;-)

Could you please give some details on batch-size? Is it possible to enable this with the EJB3 EntityManager. I'm using jboss-4.0.3RC2.

I am not using the EJB3 EntityManager or JBoss. But there is not much to it, just set the batch-size attribute on the association mapping or the table mapping. The documentation has an example for this (look for batch-size).

Basically it works like a speculative fetch. If you need the child data for one pojo it will load child data for other then known pojos, too. Just in case you might need them later. The value of batch-size determines the number of pojos for which child data records are fetched. HTH.