gistfile1.rb

class BoardingFile < DataPump

    input :hash do
      record :merchant do
        column :name, :validate => {:re => /[A-Z][a-z]+/}
      end
      records :equipments do
        column :id, :validate => {:type => Integer}
        column :name, :validate => {:re => /.{,10}/}
      end
    end

    converter do |input, output|
      input[:equipments].each do |item| 
        output[:chain] << {:id => item[:id], :equipment_name => item[:name], :merchant_name => input[:merchant][:name]}
      end
      output[:header] << {:phys_number_of_records => output[:chain].length + 1}
    end

    output :fixed_width_file do
      record :header, 'H' do
        column(:type, 1, 3, :default => 'HDR')
        column(:file_title, 4, 3, :default => 'MBF')
        column(:phys_number_of_records, 7, 2, :format => "%02d")
      end
      record :chain, 'C' do
        column(:id, 1, 7, :format => "%07d")
        column(:equipment_name, 8, 20)
        column(:merchant_name, 28, 50)
    end
end


# and then somewhere in the axiom...


exporter = BoardingFile.new(:output => File.new("result.dat"))
begin
  exporter << { :merchant => {:name => "Robert"}, :equipments => [ { :id => 1, :name => "screwdrive"}, { :id => 2, "hammer" } ] }
rescue DataPumpError => err
  err.errors.each |item| do
    # position is path for hash. eg: merchant->equipments[2]->name     
    # for file reading it would be line number and offset in the line  
    puts "Err: #{item.position}, #{item.description}" 
  end
end