Rails - Mar 2006 - validating composite model keys

I know that I should be able to figure this out by looking at the 
source, but this "ruby newbie" can''t:

I have a model for a table with a 3-column composite key. Is there a way 
to tell the stock validates_uniqueness_of() helper to validate such a 
key or do I have to write my own?

Thanks,

Larry

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