I don't know about a rails way, but this "one-liner" does the trick:
irb(main):036:0> d = Date.parse( {"(1i)"=>"2010", "(2i)"=>"8", "(3i)"=>"16"}.to_a.sort.collect{|c| c[1]}.join("-") )
=> #<Date: 4910849/2,0,2299161>
irb(main):037:0> d.to_s
=> "2010-08-16"
Or, with less magic:
h={"(1i)"=>"2010", "(2i)"=>"8", "(3i)"=>"16"}
d=Date.new(h['(1i)'].to_i, h['(2i)'].to_i, h['(3i)'].to_i)
d.to_s
=> "2010-08-16"
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