enumerable.rb

class User
  attr_accessor :name
  def initialize(name); @name = name; end
end

class Collection
  include Enumerable

  def initialize(*elements)
    @elements = elements
  end

  def each(&block)
    @elements.each(&block)
  end

  def pluck(attribute)
    map { |element| element.public_send(attribute) }
  end
end

class UserCollection < Collection
  def names
    pluck(:name)
  end
end

users = 5.times.map { |n|
  User.new("USER#{n}")
}

collection = UserCollection.new(*users)
p collection.names