Validates Uniqueness unless blank?

Hopefully this is an easy question...

I want to validate the uniqueness of a field, unless that field is
blank.. ie they didn't put anything in for it...

Any ideas? Thanks guys.

validates_uniqueness_of :field, :if Proc.new {|model| !model.field.blank?}

what does Proc.new do? I get the rest, little fuzzy on that one.

there's actually a typo there, it should be:

validates_uniqueness_of :field, :if => Proc.new {|model| !model.field.blank?}

Using Proc.new is just a short hand of using the following:

validates_uniqueness_of :field, :if => :some_field_is_not_blank?

def some_field_is_not_blank?
   !self.some_field.blank?
end

Here's some information about Proc from the Pickaxe book:

Proc objects are blocks of code that have been bound to a set of local
variables. Once
bound, the code may be called in different contexts and still access
those variables

def gen_times(factor)
return Proc.new {|n| n*factor }
end

times3 = gen_times(3)
times5 = gen_times(5)
times3.call(12) ---> 36
times5.call(5) ---> 25
times3.call(times5.call(4)) ---> 60

Awesome thanks!

I figured out that the => was missing... worked perfectly! Thanks
Adam