Mongo has a interop mechanism between replica members that lets it generate unique _id easily. But when our application wants to create an _id that starts with user_id, it becomes much harder to keep uniqueness.

I'd like to suggest to add a mechanism to be able to set an _id generation for collection as some sort of template that would look like (in my case with user_id upfront) "

", that would put my id (which is just user_id) and then mongo-generated one. Possibly more parameters could be added to make it more flexible.

What do you think?