Notes for Überdev talk about Value Objects

value_objects_and_activerecord.md

#Value Objects and ActiveRecord

##Value Objects

###Properties

• A Measure or description of something
• Equality is dependent on values rather than an identity
• Immutable

###Examples: Time, Pathname, Currency, Address

##Reference Objects vs. Value Objects ###Reference Objects

• Compared based on identity (ie primary key)

• Have mutator methods

  c1 = Customer.new(name: "George")
  c2 = Customer.new(name: "George")
  
  c1 == c2        #=> false
      
  c1.name = "Bob" #=> Updates name on customer object

###Value Objects

• Compared based on values

• Immutable

  t1 = Time.new(2013, 06, 07)
  t2 = Time.new(2013, 06, 07)
  
  t1 == t2    #=> true
      
  t1 + 5.days #=> Returns new Time object

##ActiveRecord#composed_of Note: it looks like composed_of is going to be deprecated.

Example from ActiveRecord api:

Assuming a Customer model with the attributes balance_amount and balance_currency we could create a Value Object representing the customer's balance:

class Customer < ActiveRecord::Base
  composed_of :balance, class_name: "Money", mapping: [%w(balance_amount amount), %w(balance_currency currency)]
end

Money value object:

class Money
  include Comparable
  attr_reader :amount, :currency

  DEFAULT_CURRENCY = "USD"
  EXCHANGE_RATES = {
    "USD_TO_SEK" => 6.5403,
    "SEK_TO_USD" => 0.1529
  }

  def initialize(amount, currency = DEFAULT_CURRENCY)
    @amount, @currency = amount, currency
  end

  def exchange_to(other_currency)
    exchanged_amount = (amount * EXCHANGE_RATES["#{currency}_TO_#{other_currency}"]).floor
    Money.new(exchanged_amount, other_currency)
  end

  def ==(other_money)
    amount == other_money.amount && currency == other_money.currency
  end

  def <=>(other_money)
    if currency == other_money.currency
      amount <=> other_money.amount
    else
      amount <=> other_money.exchange_to(currency).amount
    end
  end
end

##Benefits

• Moves attribute logic to a separate class.

  customer.balance.exchange_to("AUD")

• Can provide a more expressive way to compare values:

  customer.balance > Money.new(100, "SEK")

• Side-effect free

  • When a Value Object is going to be "changed," a new object should always be created instead.

##References

• ActiveRecord::Aggregations::ClassMethods
• Value Object
• Value Objects Should Be Immutable
• Values in Object Systems
• 7 Patterns to Refactor Fat ActiveRecord Models
• composed_of removal