Simple kind of event sourcing

es.rb

class StateObject
  def initialize
    @name    = nil
    @surname = nil
    @email   = nil
  end

  def update_name(name)
    @name = name
    Events::UpdateName.new(name).push
  end

  def update_surname(surname)
    @surname = surname
    Events::UpdateSurname.new(surname).push
  end

  def update_email(email)
    @email = email
    Events::UpdateEmail.new(email).push
  end

  def to_s
    [
      @name,
      @surname,
      @email
    ].join(", ")
  end

  def set_state(attribute, state)
    instance_variable_set("@#{attribute}", state)
  end
end

module Events
  class BaseEvent
    def initialize
      raise NotImplementedError
    end

    def push
      EventStore.push(self)
      state
    end

    def event_type
      raise NotImplementedError
    end

    def state
      raise NotImplementedError
    end
  end

  class UpdateName < BaseEvent
    def initialize(surname)
      @surname = surname
    end

    def attribute
      "name"
    end
    
    def state
      @surname
    end
  end

  class UpdateSurname < BaseEvent
    def initialize(surname)
      @surname = surname
    end

    def attribute
      "surname"
    end

    def state
      @surname
    end
  end

  class UpdateEmail < BaseEvent
    def initialize(email)
      @email = email
    end

    def attribute
      "email"
    end

    def state
      @email
    end
  end
end

class EventStore
  class << self
    def instance
      @instance ||= new
    end

    def push(event)
      instance.push(event)
    end

    def build
      instance.build
    end
  end

  def initialize
    @vault = []
  end

  def push(event)
    @vault << event
  end

  def build
    buildable_object = StateObject.new

    @vault.each do |event|
      buildable_object.set_state(event.attribute, event.state)
    end

    buildable_object
  end
end

user = StateObject.new
user.update_email "isqrel@gmail.com"
user.update_name "Vasilij"
user.update_surname "Melnuchyk"
user.update_name "Vasja"
user.update_name "Vasyl"
user.update_surname "Melnychuk"
user.update_email "vasilij@melnychuk.me"

puts "Old user:"
puts user

newuser = EventStore.build
puts "New user"
puts newuser