gistfile1.rb

a = []

def a.<< e
  super.tap do
    puts "Adding #{e.inspect}"
  end
end

a << 1
# Adding 1
# => [1]


class Blah
  def initialize
    @a = []
    def @a.<< e
      super.tap do
        puts "Adding #{e.inspect}"
      end
    end
  end
end

blah = Blah.new

blah.instance_eval do
  @a << 2
end

# Adding 2
# => [2]


class User
  module ProductArrayMixin
    def << e
      super.tap do
        puts "Adding #{e.inspect}"
      end
    end
  end

  attr_reader :products

  def initialize
    @products = []
    @products.extend ProductArrayMixin
  end
end

user = User.new
user.products << 3

# Adding 3
# => [3]