Non-persistent fields

Question

Hi,
I would like to use toplink with objects that contains both persistent and non-persistent fields (unmapped). When I insert such an object to the database using unit-of-work, and then try to query toplink for that same object elsewhere in my program I only get the persistent fields. I've tried using checkCacheThanDatabase() which resolved the problem but this method is only available for ReadObjectQuery not for ReadAllQuery (the javadoc is not consistent with the code).
My questions:
1. For ReadObjectQuery, can I trust the cache to always have my object, or is there a risk that the cache will drop it?
2. What to do with ReadAllQuery?
3. Any other approach that can work?

Answer 1

You can make non-persistent attributes remain with the object in a unit of work through using a clone policy and using the postMerge event.
Your class needs to implement Cloneable and in the descriptor use clone-copy-policy. This will ensure that when a cached object is registered into the unit of work the non-persistent attributes will be maintained.
When the unit of work merges a transactional object into the session cache you need to make sure you copy the non-persistent attributes from the clone to the cached object. You can do this through a DescriptorEventListener and the postMerge event.
Example:
postMerge(DescriptorEvent event) {
String tempProperty = ((MyClass) event.getObject()).getProperty();
((MyClass) event.getOringalObject()).setProperty(tempProperty);
Note that since the attribute is non-persistent it will not be guaranteed to persist indefinitely. The non-persistent attributes duration will be dependant on your caching policy, if it gets dropped from the cache, a new instance will be built from the database the next time it is accessed.
The checkCacheThanDatabase checkCacheOnly properties should not have any effect on what you get from the cache. Reads that access the database will still maintain object identity with the cache, and you should get back the correct object.