attr_object : value objects for ruby on rails

Value Objects are used to add methods on model attributes in Rails. Example, a phone_number attribute that is simply a string could have methods to return the area_code, an unformated version or a formated standardized version.

Installation

Include the gem in your Gemfile:

gem "attr_object"

Then follow these instructions to create your first attr_object :

1. Create a value object with rails generate attr_object attribute_name [type] (ex: rails generate attr_object phone fixnum). type is optional.
2. File is created in app/attr_objects (ex : app/attr_objects/phone_value.rb)
3. Add this to your model : attr_object :phone, :mobile, PhoneValue (ex : app/model/user.rb)

Use DelegateClass for you value objects (recommended)

DelegateClass will keep your value compatible with the base class of the value while giving you the opportunity to add methods specific to that class of object. Let's take a look at a PhoneValue that would return an unformated value, "323-216-3461" would become "3232163461".

class PhoneValue < DelegateClass(String)
  def unformat
    self.gsub /\D*/, ""
  end
end

The naked instance of PhoneValue("323-216-3461") still returns "323-216-3461".

If you don't want to use DelegateClass

1. Create a value object in app/values (ex : app/values/position_value.rb)
2. Add this to your model : attr_object :position, PositionValue (ex : app/model/user.rb)
3. Make sure your value object has a to_db method that returns the right type
4. (optional) Make it Comparable for sorting

class PositionValue
  include Comparable

  def <=>(other)
    to_db <=> other.to_db
  end

  def to_db
    @value.to_i
  end
end

Why use attr_object?

To explain why, I will once again use the infamous phone attribute which a String.

Slim down your models

To deal with phone numbers, we often end up with a mess of methods like this.

class User < ActiveRecord::Base
  def unformat_phone
    # ...
  end

  def format_phone
    # ...
  end

  def area_code
    # ...
  end

  private
  def phone_to_i
    # ...
  end
end

attr_object helps you clean up that mess.

class User < ActiveRecord::Base
  attr_object :phone, PhoneAttr
end

Keep responsibilities where they belong

According to the Single Responsibility Principle, methods about the phone attribute don't belong in your User model.

Rails is not just MVC

We often build applications as if we could write code in just three different places : models, views or controllers. And if things go wild, we rely on concerns and helpers. That should not be the case.