user.rb

#!/usr/bin/env ruby

require 'set'

class User
  extend Enumerable

  def self.all
    @all ||= Set.new
  end

  def self.create(attrs = {})
    all << new(attrs)
  end

  def self.each(&block)
    all.each(&block)
  end

  attr_reader :attributes

  def initialize(attributes = {})
    @attributes = attributes
  end

  def method_missing(m, *args)
    attributes[m] || super
  end
end

User.create name: "Lee"

p User.find { |u| u.name == "Lee" } #<User:0x007ff7539b4518 @attributes={:name=>"Lee"}>
p User.find { |u| u.name == "Bob" } #=> nil