Is moving documents between collections a good way to represent state changes in MongoDB?

Question

I have two collections, one (A) containing items to be processed (relatively small) and one (B) with those already processed (fairly large, wit

Answer 1

There's no way to atomically remove+save to different collections, as far as I can tell (maybe by design?)

Yes this is by design. MongoDB explicitly does not provides joins or transactions. Remove + Save is a form of transaction.

Is there a Best Practice for this situation?

You really have two low-complexity options here, both involve findAndModify.

Option #1: a single collection

Based on your description, you are basically building a queue with some extra features. If you leverage a single collection then you use findAndModify to update the status of each item as it is processing.

Unfortunately, that means you will lose this: ...that the "incoming" collection can be kept very small and fast this way.

Option #2: two collections

The other option is basically a two phase commit, leveraging findAndModify.

Take a look at the docs for this here.

Once an item is processed in A you set a field to flag it for deletion. You then copy that item over to B. Once copied to B you can then remove the item from A.