why is it that only one collection per class can have a fetch="join" on a one-to-many? how do you know when to use fetch="join" instead of fetch="select" ?

the java persistance with hibernate book states:






do i need to "enable eager fetching" or does setting fetch="join" implicitly enable eager fetching?

but i still don't know if only one collection per class is allowed to have fetch="join" ? i read that is does in Hibernate Quickly by Peak Heudecker, but that book is a couple years old so....

bump

any1?

1. Yes, fetch="join" disables lazy loading. i.e. enables eager loading. The said book says so right after the excerpt you have quoted. But there is a way to eagerly fetch without using fetch="join". You could eagerly fetch using separate selects instead of joins in the same (one) select by using fetch="select" together with lazy="false".
The best way to think about it is: 'fetch="bla" basically specifies how to perform the (eager) fetch.

2. The thing about eagerly fetching multiple collections using fetch="join" is that it could lead to the performance problems due to many joins in the same query and the implied amount of work Hibernate has to do to marshal the resultset into persistent objects and collections. You might want to read section 13.2.5 in detail. Specifically the subsection titled 'The Cartesian product problem'.