uninitialized constant ISO8583::BerlinMessage::N - ruby-on-rails

Question

I'm trying to run codes below, got from https://github.com/a2800276/8583, I put it in my /lib then try to call the BerlinMessage to test if the code was working then the error prompted. Im wondering why I unitialized constant. Thanks.
BerlinMessage.rb
require 'iso8583'
module ISO8583
class BerlinMessage < Message
mti_format N, :length => 4
mti 1100, "Authorization Request Acquirer Gateway"
mti 1110, "Authorization Request Response Issuer Gateway"
mti 1420, "Reversal Advice Acquirer Gateway"
mti 1421, "Reversal Advice Repeat Acquirer Gateway"
mti 1430, "Reversal Advice Response Issuer Gateway"
mti 1804, "Network Management Request Acquirer Gateway or Issuer Gateway"
mti 1814, "Network Management Request Response Issuer Gateway or Acquirer Gateway"
bmp 2, "Primary Account Number (PAN)", LLVAR_N, :max => 19
bmp 3, "Processing Code", N, :length => 6
bmp 4, "Amount (Transaction)", N, :length => 12
bmp 6, "Amount, Cardholder Billing" , N, :length => 12
bmp 7, "Date and Time, Transmission" , MMDDhhmmss
bmp 10, "Conversion Rate, Cardholder Billing", N, :length => 8
bmp 11, "System Trace Audit Number (STAN)", N, :length => 6
bmp 12, "Date and Time, Local Transaction", YYMMDDhhmmss
bmp 14, "Date, Expiration", YYMM
bmp 22, "POS Data Code", AN, :length => 12
bmp 23, "Card Sequence Number", N, :length => 3
bmp 24, "Function Code", N, :length => 3
bmp 25, "Message Reason Code", N, :length => 4
bmp 26, "Card Acceptor Business Code", N, :length => 4
bmp 30, "Amounts, Original", N, :length => 24
bmp 32, "Acquiring Institution Identification Code", LLVAR_N, :max => 11
bmp 35, "Track 2 Data", LLVAR_Z, :max => 37
bmp 37, "Retrieval Reference Number", ANP, :length => 12
bmp 38, "Approval Code", ANP, :length => 6
bmp 39, "Action Code", N, :length => 3
bmp 41, "Card Acceptor Terminal Identification", ANS, :length => 8
bmp 42, "Card Acceptor Identification Code", ANS, :length => 15
bmp 43, "Card Acceptor Name/Location", LLVAR_ANS, :max => 56
bmp 49, "Currency Code, Transaction", N, :length => 3
bmp 51, "Currency Code, Cardholder Billing", N, :length => 3
bmp 52, "Personal Identification Number (PIN) Data", B, :length => 8
bmp 53, "Security Related Control Information", LLVAR_B, :max => 48
bmp 54, "Amounts, Additional", LLLVAR_ANS,:max => 40
bmp 55, "Integrated Circuit Card (ICC) System Related Data", LLLVAR_B, :max => 255
bmp 56, "Original Data Elements", LLVAR_N, :max => 35
bmp 58, "Authorizing Agent Institution Identification Code", LLVAR_N, :max => 11
bmp 59, "Additional Data - Private", LLLVAR_ANS, :max => 67
bmp 64, "Message Authentication Code (MAC) Field", B, :length => 8
bmp_alias 2, :pan
bmp_alias 3, :proc_code
bmp_alias 4, :amount
bmp_alias 12, :exp_date
end
end
if __FILE__==$0
mes = ISO8583::BerlinMessage.new
mes.mti = 1110
mes[2] = 474747474747
mes["Processing Code"] = "123456"
pan = mes["Primary Account Number (PAN)"]
#mes.pan = 47474747474747
#puts mes.pan
puts mes.to_b
puts mes.to_s
#mes2 = BerlinMessage.parse input
end
After running the code I got this error,
Thanks.

Answer 1

To cite from the comment in the code you linked to explaining the mti_format method:
# the subtyped message should be told how the MTI is encoded:
#
# class MyMessage < Message
# mti_format N, :length => 4
# (...)
# end
#
# `N` above is an instance of Field which encodes numbers into their
# ASCII representations in a fixed length field. The option `length=>4`
# indicates the length of the fixed field.
Thus, you need to replace the N with the class name of your field implementation. As it stands now, ruby interprets your literal N as the name of a constant in the current class and tries to resolve it to a value. As this doesn't work, you get the error you have received.