I think it would be useful to add support for derived fields in collections.

I'm imagining it could work similarly to Excel where you can reference a cell, or group of cells, to use in a function.

The use case I want to use it for is calculating points for a sports league website. I would like to be able to use the formula `points = wins * 2 + ties` rather than having to calculate this manually. Also would like to do this for summing games played: `gamesPlayed = wins + losses + ties`.

I can see this being useful for other cases as well such as summing the number of posts for a blog.