24 Jul 2010
class Invoice < ActiveRecord::Base belongs_to :user end <%= @invoice.user.name %> <%= @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 end <%= @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.