Code:
irb(main):001:0> a = (1..20).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
irb(main):002:0> a.sort_by { rand }
=> [12, 3, 1, 19, 14, 17, 16, 13, 8, 4, 9, 5, 15, 20, 11, 10, 2, 7, 6, 18]
irb(main):003:0> a.sort_by { rand }
=> [14, 9, 19, 3, 20, 13, 16, 12, 2, 11, 6, 17, 1, 8, 5, 15, 10, 18, 7, 4]
irb(main):004:0> a.sort_by { rand }
=> [20, 12, 5, 14, 11, 3, 17, 6, 1, 13, 4, 15, 2, 18, 10, 8, 19, 16, 7, 9]
irb(main):005:0>
Just grab the first element of the shuffled array.