composite Unique Keys?

[Jason Toy]

I have several different document types that I store.  I use a serialized
integer that is unique to the document type.  If I use id as the uniqueKey,
then there is a possibility to have colliding docs on the id, what would be
the best way to have a unique id given I am storing my unique identifier in
an integer field? I've seen other solutions where the unique is consists of
"DocTypeString 123245"  as the id which seems pretty inefficient to me.