Using Null Object pattern in Ruby on Rails apps

null_object_example.rb

# E.g. for a banking app where users might not have bank accounts yet.
# Not having accounts could mean lots of code like this:

if user.savings_account.present?
    user.savings_account.available_balance_cents
end

# But with the NullObject pattern I can confidently call:
user.savings_account.available_balance_cents


# because in the background my model has me covered:
class User < ApplicationRecord
 
  def savings_account
    self.accounts.where(savings: true).first || NullAccount.new
  end
 
end

# domain/null_account.rb
class NullAccount

  def id
    raise "You are calling id on an account that doesn't exist.  Make sure the user you're working with has accounts first before trying to call them."
  end

  def available_balance
    Money.new(available_balance_cents, available_balance_currency)
  end

  def available_balance_cents
    0
  end

  def available_balance_currency
    "CAD"
  end

  def valid?
    false
  end

  def present?
    false
  end

  def blank?
    true
  end

  def account_key
    raise "account_key is being called on a NullAccount!!"
  end

  def currency
    "CAD"
  end

  def save
    raise ActiveRecord::RecordInvalid
  end


  def update
    raise ActiveRecord::RecordInvalid
  end
end

# This pattern can be extended for other use cases, such as representing a fake user.  
# E.g. when working with the papertrail gem that records who makes database changes, 
# you can create a `SystemUser` as a fake user to represent changes to data made by scheduled jobs, data migrations etc