covered_by_unique_index?

# File activerecord/lib/active_record/validations/uniqueness.rb, line 82
      def covered_by_unique_index?(klass, record, attribute, scope)
        @covered ||= self.attributes.map(&:to_s).select do |attr|
          attributes = scope + [attr]
          attributes = resolve_attributes(record, attributes)

          klass.connection.schema_cache.indexes(klass.table_name).any? do |index|
            index.unique &&
              index.where.nil? &&
              (Array(index.columns) - attributes).empty?
          end
        end

        @covered.include?(attribute.to_s)
      end