Rails - Jun 2006 - How to validate uniqueness only if value exists

I''m looking for the best way to use validation to confirm that an 
optional field (hostname) is unique only if a value is entered in the 
form.

I was originally following the path shown in Agile Web Development with 
Rails first edition, page 66, to define the "validate" method, but 
didn''t have a clue how to include a uniqueness test there.  Right now,
I
have the following and it seems to be working as intended:

validates_uniqueness_of :hostname, :if => :hostname?

I guess I''m looking for some validation myself that this is a good way 
to handle this.  I''d also love to hear of other ways to make this work.

Thanks.

-Mason

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