Spreadsheet example.rb

def errors_report
  return nil unless self.source.present?

  # self.source — the paperclip attachment
  book = Spreadsheet.open(self.source.path)
  sheet = book.worksheet(0)

  errors = self.import_errors.map { |e| e[:line] }
  path = "tmp/#{self.source.original_filename}-errors.xls"

  # debugger
  # sheet.rows
  # => []

  # I need to remove all the rows which is not included in the errors array
  sheet.each do |row|
    # this block executing correctly
    next if errors.include?(row.idx)
    sheet.delete_row(row.idx)
  end

  return path if book.write(path)
  # => got the same file as on input
  nil
end

Don't save the file back to the same file name if you edit an XLS.