Parameterized custom mappings?

Question

Hi all,
Is there a way to define custom mappings which are implemented by a parameterized query, the parameter of which can be set at runtime?
I know that there are transformation mappings, and that you can implement your own custom queries to realize mappings, but there is no way to send a parameter value from the application to the query at runtime. The parameters are either limited to the database fields which are already involved in the mapping (in the case of custom query for mapping), or must be extracted from the object context of the mapping (in the case of transformation mapping).
I also know that you can define a "Map" instance as the collection type for a one-to-many or many-to-many mapping, but afaik this involves loading the whole set into the session before an object can be queried by key, even if transparent indirection through the "IndirectMap" interface is in place...
What I am looking for is a kind of ValueHolder that can take parameters which define the value loaded into the ValueHolder.
Is there a way to do this?
What I want to implement is the following scenario:
A class "Organization" contains a method "User getUser(String login)". The parameterized getUser operation should be implemented cleanly with a TopLink mapping, not through an external ReadObjectQuery, since the session context of the operation is not known at implementation time and should be managed by TopLink, in the same way as other pre-defined mappings are.
Any ideas?
Thanks,
Mirko
Message was edited by:
jRipple

Answer 1

Since nobody else seems to have a solution, i at least want to explain the approach I followed now to solve the problem, but it isn't a clean one...
I defined a "Map" collection property and put a TransformationMapping on it. The buildAttributeValue oft the mapping is implemented to return a homemade "Map" implementation which uses the session given to it from the buildAttributeValue "session" argument to retrieve the specific value mapped to a key via a ReadObjectQuery whenever Map.get(Object key) is called.
This basically works in my application, but I wouldn't suggest to completely rely on that carelessly, because the session given on Map construction might have become invalid some time after the attribute value has been built and before an object ist retrieved via Map.get(). An ordinary one-to-many collection mapping would still work in this case, since the whole collection is already loaded into memory. My own collection, however, would throw an exception, since it uses an invalid session object to execute a query on.
TopLink seems to have a design principle saying that persistent objects never should hold a reference to the session they are registered in, but this principle is violated by my solution...