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compose objects without initializing objects that are not in hash

Your question raises some interesting issues. I will try to explain how you can fix it, but, as @Uri mentions, there may be better ways to address your problem.

I've assumed @tranfee is to be set equal to the first value in the hash whose key begins with "tran" and that @rate is to be set equal to the first value in the hash whose key begins with "rate". If that interpretation is not correct, please let me know.

Note that I've put initialize in the PaymentType module in a class (Papa) and made TranFee and Rate subclasses. That's the only way you can use super within initialize in the subclasses of that class.


class Transaction
  attr_reader :tranfee, :rate

  def initialize(hash={})
    o =
    @tranfee =
    o =
    @rate =


module PaymentType
  class Papa
    def initialize(hash, prefix)
      key, value = hash.find { |key,value|
key.start_with?(prefix) && value }
      (raise ArgumentError, "No key beginning with
#{prefix}") unless key
      instance_variable_set("@#{key}", value)
      self.class.singleton_class.class_eval {
attr_reader key }

  class TranFee < Papa
    def initialize(hash)
      super hash, "tran"

  class Rate < Papa
    def initialize(hash)
      super hash, "rate"

I believe the method Object#singleton_class has been available since Ruby 1.9.3.


reg_debit = {"name" => "reg_debit",
"rate_base" => 0.0005, "tran_fee" => 0.21,
             "rate_basis_points" => 0.002,
"tran_auth_fee" => 0.10}

a = reg_debit
p Transaction.instance_methods(false) #=>
[:tranfee, :rate]
p a.instance_variables                #=>
[:@tranfee, :@rate]
p a.tranfee                           #=> 0.21
p a.rate                              #=>

Categories : Ruby

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