One thing I love Rails is it provides really handy URL helpers to the developers. Today, I am going to present you guys how to use polymorphic_url like a ninja.

Say, I’ve got the following models:

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# app/models/vehicle.rb
class Vehicle < ActiveRecord::Base
  ...
end

# app/models/bicycle.rb
class Bicycle < Vehicle
  ...
end

# app/models/motercycle.rb
class Motorcycle < Vehicle
  ...
end

As you can see, we’ve have a STI table named vehicles in the database, and it stores all the bicycles, motorcycles and the other new vehicles we may have in the future, like unicycle.

We have a collection of vehicles, @vehicles, it contains bicycles and motercycles. What we are going to do is iterating the @vehicles, and display a link to the vehicle detail page. With polymorphic_url, we can write down the following code:

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<% @vechicles.each do |v| %>
  <%= link_to "Vehicle Detail", polymorphic_url(v) %>
<% end %>

instead of:

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<% @vehicles.each do |v| %>
  <% if v.is_a? Bicycle %>
      <%= link_to "Vehicle Detail", bicycle_page(v) %>
  <% elsif v.is_a? Motercycle %>
     <%= link_to "Vehicle Detail", motorcycle_page(v) %>
  <% end %>
<% end %>

Once we add a new kind of vehicle, we don’t need to open the above view file and add another elsif, it complies to the Open Closed Principle perfectly!

The request to bicycle detail will go to:

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# app/controllers/bicycles_controller.rb
class BicyclesController < ApplicationController
  def show
    @bicycle = Bicycle.find(params[:id])
  end
end

# app/views/bicycles/show.html.erb
<h1>Here comes the bicycle details</h1>

And the request to motorcycle detail will go to:

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# app/controllers/motorcycles_controller.rb
class MotorcyclesController < ApplicationController
  def show
    @motorcycle = Motorcycle.find(params[:id])
  end
end

# app/views/motorcycles/show.html.erb
<h1>Here comes the motorcycle details</h1>

What if we want to have the same controller rendering the same view for all the vehicles? Say, we only want one VehiclesController#show for all the vehicles’s detail, here we go:

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# app/controllers/vehicles_controller.rb
class VechiclesController < ApplicationController
  def show
    @vehicle = Vehicle.find(params[:id])
  end
end

# app/views/vechicles/show.html.erb
<h1>Here comes the motorcycle details</h1>

You may say we can just use vehicle_url(v) to generate the URL, I can not agree more, but we are exploring something deep inside of Ruby on Rails, so, bare with me, :)

To have the polymorphic_url, generate the URL like vechicle_url, we need to overwrite self.model_name for the Vehicle, here’s how I found it out:

1. the polymorphic_url will call build_named_route_call to generate the URL
2. the build_named_route_call will call RecordIdentifier.__send__("plural_class_name", record) to find out reource name, say, a record of Bicycle will generate bicycles
3. the plural_class_name will call model_name_from_record_or_class to determine the model name of the record
4. model_name_from_record_or_class will call the record’s class model_name method to find out the model name of the class

We need to override the self.model_name to let polymorphic_url pick up the right model name:

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# app/models/vehicle.rb
class Vehicle < ActiveRecord::Base
  ...

  def self.model_name
    ActiveModel::Name.new(self, nil, "Vehicle")
  end
end

If we want a custom URL and view template for specific vehicle, we can also write something like:

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# app/models/unicycle.rb
class Unicycle < Vehicle
  def self.model_name
    ActiveModel::Name.new(self, nil, "Unicycle")
  end
end

With the following customization, polymorphic_url(@unicycle) will generate the URL unicycle_url(@unicycle).