Delegate in Rails

A model should only talk to its immediate association. According to the Law of Demeter, you shouldn’t talk to the association’s property or association’s association.

Here we talk about Rails delegate approach.

BAD SMELL

class Profile < ActiveRecord::Base
belongs_to :user
end

<%= @profile.user.address %>
<%= @profile.user.city %>

Rails provides helper method to accomplish delegation, which uses DSL to generate wrapper methods. It can also prevent error call method on nil object if :allow_nil => true is added.

Refactored

class Profile < ActiveRecord::Base
  belongs_to :user
  delegate :address, :city, :to => :user, :prefix => true, :allow_nil => true
end
<%= @profile.user_address %>
<%= @profile.user_city %>

Originally published at Kolosek blog.