(this snippet was tested with ruby 2.3)

RUBY helps a lot to write DRY code.
This feature is both great and very disturbing.

ruby on Rails, for example, is full of meta-programming.
That's how the rails "magic" happens.
Let's point out it's not magicall, and pretty simple to explain.

NB : This snippet requires some basic understanding of Object Oriented Programming.
I tried to take baby steps to explain it.
It should be very easy to understand.

Meta-programming, what is it ?

Let's assume we play with different living-dead species : Zombies and Vampires, Werevamps. Vampires have appetite for blood, whereas Zombies have appetite for brains, and Werevamps crave for blood and/or flesh.

We would like to be able to define a "macro" that let's us define what a class has appetite for, so we could write :

class Zombie < LivingDead
    has_appetite_for :brains
end

class Vampire < LivingDead
    has_appetite_for :blood
end

class Werevamp < LivingDead
    has_appetite_for :blood
    has_appetite_for :flesh
end

bob = Werevamp.new

bob.regime 
#=> [:blood, :flesh]

bob.hunt_blood
#=> "bob is hunting for blood"

bob.hunt_flesh
#=> "bob is hunting for flesh"

The "regime, " methods exists just because Werevamp is derived from LivingDead.
But how do we make that happening ?
The answer is "Meta-programming", also known as "macro".

This is precisely what rails classes are doing beind the hood. It's no magic.
For instance, your has_many :persons in your Company model gives you methods like Company.first.persons wich returns a collection of Person objects. Let's see how this is done, step by step

Monkey patching an object

We know how to monkey patch an object :

fred = "Zombie"
#=> "Zombie"

fred.class
#=> String

def fred.has_appetite_for
"brains"
end  
#=> :has_appetite_for

fred.has_appetite_for
#=> "brains"

We monkey patched the fred object.
It now has a new method called :has_appetite_for

fred is an object. It's a simple String.
Since a class is an object of class Class, we could do the same on the class itself.

Patching a class

class Zombie
end  
#=> nil

Zombie.class
#=> Class

my_zombie_classs = Zombie
#=> Zombie

my_zombie_class.class
#=> Class 

def my_zombie_class.has_appetite_for
     "brain"
end
#=> :has_appetite_for

Zombie.has_appetite_for
#=> "brain"

Pushing it into the class

We could put that the class itself.
Let's do that with the Vampire class

class Vampire
    def self.has_appetite_for
        "blood"
    end

    self.has_appetite_for
end
#=> "blood"

Vampire.has_appetite_for
#=> "blood"

Within the class definition, self refers to the Vampire class itself. Therefore, we could replace self with Vampire in the above example, it would do the same thing.

Now we would like to have a method called "has_appetite_for" that gets the food as an argument. Let's assume we have a new kind of living-dead called "Werevamp". A Werevamp has taste for either blood or flesh.

class Werevamp

    def self.has_appetite_for(food) 
        puts "#{self} has appetite for #{food}"
    end

    has_appetite_for :flesh
    has_appetite_for :blood
end

#=> "Werevamp has appetite for flesh"
#=> "Werevamp has appetite for blood"

bob = Werevamp.new
#=> "Werevamp has appetite for flesh"
#=> "Werevamp has appetite for blood"

If we remove the "self" call, ruby assumes that has_appetite_for is called on self

One last step : define a method within our method : to do that we use
define_method

class Werevamp

  def self.has_appetite_for(name) 
    puts "#{self} has appetite for #{name}"
    define_method("hunts_#{name}") do
      puts "A #{self.class} is now hunting for #{name}"
    end
  end

  has_appetite_for :flesh
  has_appetite_for :blood
end

#=> "Werevamp has appetite for flesh"
#=> "Werevamp has appetite for blood"
#=> => :hunts_blood

bob = Werevamp.new
#=> => #<Werevamp:0x00000001e244c8>

bob.hunts_blood
#=> A Werevamp is now hunting for blood
#=> =>nil               
#   reminder : puts returns nil

Pushing it on a parent class

Now we have everything we need to push the behavior a parent class.

class LivingDead
  attr :name, :food    # this too is meta-programming, by the way.  
  def initialize(name)
    @name = name
  end

  def self.has_appetite_for(food_name)
    puts "#{name} has appetite for #{food_name}"
    define_method("hunts_#{food_name}") do
      puts "#{self.name} is hunding for #{food_name}"
    end
  end

end

class Zombie < LivingDead
  has_appetite_for :brain
end

john = Zombie.new("J")
#=> => #<Zombie:0x00000002ad8610 @name="J">

john.hunts_brain
#=> J is hunting for brain
#=> => [:brain]

Important note :

One does NEED to understand the meaning of self here.

When ruby executes code, the self variable is contextual.

Wich means that in the class definition, self refers to the class itself.

In a class definition, self.class is a Class.

While when the code is being executed on the instance, self refers to the instance.

Let's push that to a module

Now we can create a LivingDead module.
Let's add a feature : we want to keep track of who eats what.

module LivingDead
  attr :regime
  @regime = a = Hash.new{ |hash, key| hash[key] = [] }

  def self.add_to_regime name, sender
    @regime["#{sender}"] << name
  end

  def self.get_regime sender
    @regime["#{sender}"]
  end

  private
  def self.regimes
    @regimes
  end

  public
  class Base
    include LivingDead

    attr :name; :regime

    def initialize(name)
      @name = name
    end

    def self.has_appetite_for(food_name)

      puts "#{name} has appetite for #{food_name}"
      # send :add_to_regime, food_name
      LivingDead::add_to_regime food_name, self

      define_method("hunts_#{food_name}") do
        puts "#{name} is now hunting for #{food_name}"
      end

      define_method (:get_regime) do
        LivingDead::get_regime "#{self.class}"
      end
    end

    def regimes
      LivingDead::regime
    end

  end
end

class Zombie < LivingDead::Base
  has_appetite_for :brain
end

class Vampire < LivingDead::Base
  has_appetite_for :blood
end

class Werevamp < LivingDead::Base
  has_appetite_for :flesh
  has_appetite_for :blood
end

john = Zombie.new("John")
bob = Vampire.new("Bob")
jenny = Werevamp.new("Jenny")

john.get_regime
#=> => [:brain]

bob.get_regime
#=> => [:blood]
jenny.get_regime
#=> => [:flesh, :blood]

john.hunts_brain

bob.hunts_blood
jenny.hunts_flesh

include LivingDead
LivingDead::regime
#=> => {"Zombie"=>[:brain], "Vampire"=>[:blood], "Werevamp"=>[:flesh, :blood]}

jenny.regimes
#=> => {"Zombie"=>[:brain], "Vampire"=>[:blood], "Werevamp"=>[:flesh, :blood]}

LivingDead::regimes
#=> => nil   because private !

Important note :
See how the "self" is behaving. When it's interpreted as a code from the class, it's the class. When it's interpreted from within the object, it's the object name.

I included a private sample so we can also see here that it does work as expected.

A good example of the above note :

     define_method (:get_regime) do
      LivingDead::get_regime "#{self.class}"
     end   

We have to consider here that the "self" will be interpreted WITHIN the object.
Therefore, the self.class is now being used.