Question

I’m reading a book called RailsAntiPatterns. In the converter method below, a new OrderConverter object is instantiated, and I assume self refers to an instance of Order class.

# app/models/order.rb
class Order < ActiveRecord::Base
  def converter 
    OrderConverter.new(self)
  end 

end

# app/models/order_converter.rb 
class OrderConverter

  attr_reader :order 

  def initialize(order)
    @order = order 
  end

  def to_xml # ...
  end

  def to_json # ...
  end

  def to_csv # ...
  end

  def to_pdf # ...
  end 
end

1. Why instantiate a new class inside of converter?
2. Why does self need to be passed as an argument?
3. Can you summarize in lay terms what’s going on?

Answer 1

Why instantiate a new class inside of converter?

Of course, it’s up to the choice of the author, but it’s probably convenient. For instance:

@my_order = Order.new 
@my_order.converter.to_xml

That reads quite nicely, which is important in the eyes of a Rubyist. As the original designer of Ruby, Yukihiro Matsumoto (Matz) has said:

But in fact we need to focus on humans, on how humans care about doing programming or operating the application of the machines. We are the masters. They are the slaves.

Readibility for humans is, therefore, important if you wish to produce elegant Ruby code.

Why does “self” need to be passed as an argument?

Quite simply, OrderConverter requires an order to convert. Since the method converter is defined for instances of the Order class, an instance that wishes to convert itself will pass self as the argument to OrderConverter#new.

can you summarize in lay terms what’s going on?

I hope the above has done that for you.