Thoughts on a cosntraint ?

I have a table that consists of 3 columns.

vendor_key
mfg_key
preferred (boolean)

The idea is that a given vendor is the vendor we want to use for each
manufacturer for a given project.

I need to constrain such that  only on row of mfg, vendor and project can
be set to TRUE.

I would be interested in seeing other peoples approaches on this, also.

Any thoughts? 
-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                        -- Benjamin Franklin