Value objects

02 Mar 2019

A small simple object, like money or a date range, whose equality isn’t based on identity. Martin Fowler

Symbol, String, Integer and Range are example of value objects.

Value Objects in Ruby on Rails

Notes to this presentation.

March is not 3. 3 is not March. March is March.

March is a range of days, 1st – 31st.

Examples of value objects:

Date, Time
Weight, Height
Duration
Temperature
Address
Money

A value object has:

Identity based on state
- a dollar is a dollar
- Nov 5 is Nov 5
- 98.6F is 98.6F
- 10 Downing Street is 10 Downing Street
Equality based on type and state
- 98.6F != $98.60

They are immutable – or should be treated as such – and typically do not have setters.

Scalar values example

class Patient
  attr_accessor :height, :weight
end

patient.height #=> 65
patient.weight #=> 90

What do these values represent? What are their units?

class Height
  def initialize(inches)
    @inches = inches
  end

  def inches
    @inches
  end

  def centimetres
    inches * 2.54
  end
end

class Weight
  def initialize(pounds)
    @pounds = pounds
  end

  def pounds
    @pounds
  end

  def kilograms
    pounds / 2.2
  end
end

class Patient
  attr_accessor :height_inches, :weight_pounds

  def height
    Height.new(@height_inches)
  end

  def weight
    Weight.new(@weight_pounds)
  end
end

patient.weight.pounds #=> 100
patient.weight.kilograms #=> 45.4545454545

patient.height.inches #=> 65
patient.height.centimetres #=> 165.1

Equality

a = Weight.new(100)
b = Weight.new(110)
c = Weight.new(100)

a == b #=> false
a == c #=> false

a and c are not equal because they are different objects.

class Weight
  # ...

  def hash
    pounds.hash
  end

  def eql?(other)
    self.class == other.class &&
      self.pounds == other.pounds
  end
  alias :== eql?
end

We need to override #hash when we override #==.

The weight class now looks like:

  attr_reader :pounds

  def initialize(pounds)
    @pounds = pounds
  end

  def kilograms
    pounds / 2.2
  end

  def hash
    pounds.hash
  end

  def eql?(other)
    self.class == other.class &&
      self.pounds == other.pounds
  end
  alias :== eql?
end

Refactor to a struct

Weight = Struct.new(:pounds) do
  def kilograms
    pounts/2.2
  end
end

a = Weight.new(100)
b = Weight.new(110)
c = Weight.new(100)

a == b #=> false
a == c #=> true

But structs are mutable.

Value object gem

You can use value_object gem.

class Timespan < ValueObject::Base
  has_fields :start_time, :end_time

  def duration
    Duration.new(end_time - start_time)
  end

  def contains?(time)
    (start_time..end_time).cover?(time)
  end

  def overlays?(other)
    contains?(other.start_time) || contains?(other.end_time)
  end
end