Comparing txid_current() to xmin

I am trying to make a trigger that updates a row once and only once per
transaction (even if this trigger gets fired multiple times). The general
idea is that for a user we have a version number. When we modify the
user's data, the version number is incremented then set on the object. We
only need to increment the version number once.

I am thinking about doing something like:

update user
set version=version+1
where txid_current() != xmin and user_id = 352395;

So I guess my questions are:

How dirty is this?
Will I run into issues?
Is there a better way of doing this?

Thanks,
Mike

--
Michael Lewis
lolrus.org
mikelikespie(at)gmail(dot)com