I am working on a simple messaging system over mongodb. 

Suppose I am user A, having private messages from B and C: in my inbox view I would like to be able to show the last message from each, either sent from me or from B or C;

take the facebook messaging page for instance. That's what I'd like to do.

Currently I have a field called "last.message" where I store the last message sent by any of the two people but, is there a better way to implement this?

Thanks in advance.