Fix id_partition interpolation with BSON::ObjectId

[jarthod]

Hey all 👋

I noticed recently that id_partition interpolation was not working any more in my project, it was always nil.
As I investigated I noticed the reason is quite simple, the code expects the model instance id to be a String, whereas it's a BSON::ObjectId (and it has been so for a while in mongoid).

I suspect it was probably a String in earlier version of mongoid? At some point it was working fine in my code because I have old folders with the id_partition but I can't tell exactly when it stopped working because I delete old files regularly.

Anyway here is the fix I suggest (with an addition test for that): I added BSON::ObjectId to the supported types and .to_s to convert it to a String before the scan. (.to_s is a no-op if already a String). I tested this fix in my project and it restored the working id_partition interpolation.

New test before my fix showing the problem:

Let me know if you have any questions!

[jarthod]

BTW one thing I noticed here also is that this id_partition is pretty long and costs a lot of folders (inodes), unless you have a number of records that no machine can even store, you'll create a lot of unique folders containing only one record. paperclip default (for string ids) is to take the first 3 groups of 3 letters: https://github.com/kreeti/kt-paperclip/blob/master/lib/paperclip/interpolations.rb#L186 which means only 3 levels of folders and is a more sensible default IMO for the average use-case.

WDYT about changing this? as the behavior was broken and we're fixing it maybe it's a good time to change the default at the same time so people only have to deal with this change once?