updating only new members of has_many assocation

[rez]

If I had two models message_thread and message in a has_many association, what is the best way to pull only new data from a json proxy

ie.

client store: message_thread with 43 messages
server: message_thread with 47 messages

I only want to pull over the wire the 4 new messages to sync the store, not the entire message_thread


Thanks,
Rob