migrate uniqueness validator to the new DSL

lib/goo/validators/implementations/unique.rb

@@ -0,0 +1,20 @@
+module Goo
+  module Validators
+    class Unique < ValidatorBase
+      include Validator
+
+      key :unique
+
+      error_message ->(obj) { "`#{@attr}` must be unique. " +
+        "There are other model instances with the same attribute value `#{@value}`."}
+
+      validity_check -> (obj) do
+        return true if @value.nil?
+
+        !Goo::SPARQL::Queries.duplicate_attribute_value?(@inst,@attr)
+      end
+
+
+    end
+  end
+end