Summary: Is there a recommended way of ensuring each document has a unique integer assigned to it?

Explanation: I've got an existing interface tailored to an existing MySQL DB (that I have no control over). I need to implement this interface for MongoDB (that I have complete control over). The PK for a table is INTEGER in MySQL and thus the interface for things like getEntryById take an integer.

I figure I'll just have MongoDB generate a unique integer for property userId (the name of the PK in the MySQL table) but then still use _id for the PK (of type ObjectId).

Is there a recommended way of generating unique numbers like that? Obviously I could do something like max(allUserIds) 1 but is there a better way? There must be.

Edit Re-formated for clarity and put my question up front before explaining.