The Law of Demeter for ruby on rails

According to the law of Demeter, a model should only talk to its immediate association, don’t talk to the association’s association and association’s property, it is a case of loose coupling.

Bad Smell

  class Invoice < ActiveRecord::Base
    belongs_to :user

  <%= %>
  <%= @invoice.user.address %>
  <%= @invoice.user.cellphone %>

In this example, invoice model calls the association(user)’s property(name, address, and cellphone), which violates the law of Demeter. We should add some wrapper methods.


  class Invoice < ActiveRecord::Base
    belongs_to :user
    delegate :name, :address, :cellphone, :to => :user, :prefix => true

  <%= @invoice.user_name %>
  <%= @invoice.user_address %>
  <%= @invoice.user_cellphone %>

Luckily, Rails provides a helper method delegate which utilizes the DSL way to generates the wrapper methods. Besides the loose coupling, delegate also prevents the error call method on nil object if you add option :allow_nil => true.