validation_needed?

# File activerecord/lib/active_record/validations/uniqueness.rb, line 69
      def validation_needed?(klass, record, attribute)
        return true if options[:conditions] || options.key?(:case_sensitive)

        scope = Array(options[:scope])
        attributes = scope + [attribute]
        attributes = resolve_attributes(record, attributes)

        return true if attributes.any? { |attr| record.attribute_changed?(attr) ||
                                                record.read_attribute(attr).nil? }

        !covered_by_unique_index?(klass, record, attribute, scope)
      end