def must_have_only_one_current_quiz
      errors.add(:is_current, 'should have only one current status') if self.is_current_equals(true).length > 1
    end

>> Location.country_equals('Indonesia').length 
=> 3
>> cq = Quiz.new(:name => 'ano', :is_current => true)
=> #<Quiz id: nil, name: "ano", created_at: nil, updated_at: nil, is_current: true>
>> cq.save
NoMethodError: undefined method `is_current_equals' for #<Quiz:0x10339e1b8>