Define Method

We use define method to create a method on the fly given a string as a name.

  class A
    define_method :c do
      puts "Hey!"
    end
  end

  A.new.b

  A.new.c(1,2)

Exercise - Green Hamsters

Hamsters come in red, green, blue and orange.

A hamster class looks like this:

  class Hamster
    attr_accessor :colour
  end

for now we’ll just assume that only these colours are allowed.

Given an instance of hamster, I would like to be able to call something like:

  hammy = Hamster.new
  hammy.colour = :red
  hammy.is_red?

and have it return true or false

Bonus marks

Rather than defining a limited set of methods in advance, use method_missing to catch when a method is not defined, then define it on the fly. If I call is_taupe? method_missing should catch that, define a Hanster#is_taupe? method, then call it.

Exercise - drying up code

The following real code is full of duplication. How might you use define_method to dry it up?

  class Widget
    def product
      product = Product.find_by_slug(object_slug)
      if !product
        product = Product.first
      end
      product
    end

    def poem
      poem = Poem.find_by_slug(object_slug)
      if !poem
        poem = Poem.first
      end
      poem
    end
  end

If you don’t want to create a whole Rails instance, you can use a simple scaffold like this:

  class Product
    class << self
      def find_by_slug
        return "success"
      end
    end
  end
  class Poem < Product; end