Example of using Ruby's Meta-programming to build finders and reduce duplication.

sales_engine.rb

module HasAttributes

  def attribute(name) #id
    # With the given name add an instance method with that name
    # define_method(name) do
    #   instance_variable_get("@#{name}")
    # end

    attr_reader name
    # With the given name add a class method find_by_NAME

    define_singleton_method("find_by_#{name}") do |value|
      all.find_all do |object|
        object.send(name) == value
      end
    end
  end

end

class Customer
  extend HasAttributes

  def self.all
    # go load from the csv
    []
  end

  attribute :id
  attribute :first_name
  attribute :last_name
end

class Merchant
  extend HasAttributes

  def self.all
    # go load from the csv
    []
  end

  attribute :id
  attribute :name

end