Uniqueness Validation With Ripple (Riak On Rails)

I’m a riak newbie who is trying to implement Riak On Rails using Ripple as modeling layer. But while creating User model, I wanted validation for uniqueness of user. Validation such as presence is available with Ripple but I couldn’t find proper validation method directly provided for the uniqueness. So, below is the sample code of User model I developed for validation.
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