that will return nil if the user doesn't exist.
If you don't need to load the friend object into memory, sure, you
could do a count instead of a find.
There's really nothing intensive about this, assuming you have indexes
setup. Considering that one user should probably only be able to be
friends with another user once, you can create a unique index, which
will be very zippy.
Also, be on the lookout for signs that you might want to promote a
Friendship to its own abstraction.