To be honest I feel like I saw this mentioned somewhere, but after looking around I couldn't find it, so I'll just post this anyway.
I'd like the ability to set a property as the associative key on ArrayCollections. This functionality could probably be done in a PostLoad by iterating over the objects, but it seems like ArrayCollection should be able to handle this functionality already, it's just a matter of modifying the mapper/parser.
If you were able to specify an ArrayCollection key column then you would be able to have functions like getParam without having to loop over the objects in the collection. Something like: