Something I've realized doing this:
It could be possible to do it differently, but ideally each "parent" {eg. board} would have references to its children {eg. thread}, but not the other way around.
So a board database object contains all the information about which threads it contains, but threads objects don't require any information about which board they are part of. Everything just references it's children via hashes/url.
On the flip side if you did decide to have a thread contain information about it's parent board, you could then use the thread to go up the tree and find other threads in the same board. Interesting but not fundamentally necessarily for a basic implementation.