Rails - Oct 2008 - Manipulating/Accessing objects that are :polymorphic => true

I am working on an application to track warranty registrations for my
employers product line.  The product line includes the following
products:

spreaders
snow_plows
skid_steers

Each of these products have different information that I have to track.
The tables would look something line this:

    create_table :spreaders do |t|
      t.integer :registration_id
      t.string :spreader_sn
      t.string :spreader_model_number

    create_table :snow_plows do |t|
      t.integer :registration_id
      t.string :power_pack_sn
      t.string :control_package_sn
      t.string :blade_sn

    create_table :skid_steers do |t|
      t.integer :registration_id
      t.string :plow_sn
      t.string :plow_control_sn
      t.string :blade_sn

Notice how each of these products need to collect different information.
They are all registrations so I can create table to hold these
registrations with information that is common to them all.

    create_table :registrations do |t|
      t.integer :customer_id
      t.integer :dealer_id
      t.date :date_purchased
      t.integer :vehicle_id
      t.text :comment
      t.integer :registerable_id
      t.string :registerable_type

I have the models defined like so:

    class Registration < ActiveRecord::Base
      belongs_to :customer
      belongs_to :dealer
      belongs_to :vehicle
      belongs_to :registerable, :polymorphic => true
    end

    class Spreader < ActiveRecord::Base
      has_many :registrations, :as => :registerable
    end

    class SnowPlow < ActiveRecord::Base
      has_many :registrations, :as => :registerable
    end

    class SkidSteer < ActiveRecord::Base
      has_many :registrations, :as => :registerable
    end

At this point I am trying to simply display a record on the registration
view and can''t seem to understand how I would access this. 
Here''s my
registration view:

1  <h1>Listing registrations</h1>
2
3  <table>
4    <tr>
5      <th>Customer Name</th>
6      <th>Dealer Name</th>
7      <th>Date purchased</th>
8      <th>Product</th>
9      <th>Product SN</th>
10     <th>Vehicle</th>
11     <th>Comment</th>
12   </tr>
13
14   <% for registration in @registrations %>
15   <tr>
16     <td><%=h registration.customer.full_name %></td>
17     <td><%=h registration.dealer.dealer_name %></td>
18     <td><%=h registration.date_purchased %></td>
19     <td><%=h registration.registerable_type %>
20     <td><%= registration.registerable_type.id %>
21     <td><%=h registration.vehicle.model %></td>
22     <td><%=h registration.comment %></td>
23   </tr>
24   <% end %>
25 </table>
26
27 <%= link_to ''New Registration'', new_registration_path
%>

On lines 16, 17 and 21, I am able to display the full_name of the
Customer, likely because my Registration belongs_to the Customer.  I am
also able to see the dealer_name of the Dealer, similarly because my
Registration belongs_to the Dealer.  And finally the model of the
Vehicle, again because of the magical belongs_to declaration.
Wonderful.  On line 20 is where I loose my mind.

I''m thinking there should be some similar magic going on behind the
scenes to view the product (Spreader, SnowPlow, or SkidSteer) of the
type listed in the registerable_type field because of my nifty:
    belongs_to :registerable, :polymorphic => true
declaration in the Registration model.  How would I access this record?
It seems to me this functionality needs to be customized in the
RegistrationsController because unlike the simple foreign key
relationship Customers and Dealers enjoy with Registrations Rails does
not know how to handle this polymorphic relationship by default.  Or,
which is probably more likely, I have no idea what I''m talking about
and
Rails DOES magically pull in the correct table because of that
association.  Ideas?

I can understand how to do this if I were to do this entirely
differently by changing my design.  For instance creating three new
models called SpreaderRegistration, SnowPlowRegistration and
SkidSteerRegistration and just throwing the whole idea of polymorphic
relationships out the window.  But it introduces duplication and I would
have to maintain three separate MVC''s for each.  Seems silly and not at
all extensible.  What if I have to add a new product that needs to be
registered?  Not I good idea I think.

There are other considerations I need to think about as well; such as
how would I tell the Registration it is registerable_type Spreader so it
can create a new registration form that loads in the fields I need for
the Spreader as well as the fields for all registrations.  I am getting
ahead of myself.

This MUST be a common problem.  Can someone point me to a good tutorial
that deals with this?  I''ve found others that talk about polymorphic
types and give migration and model examples, but nothing for the view or
controller.

Thanks for your thoughts and time.
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I am having the same problem in relation to your last point:


 "There are other considerations I need to think about as well; such as
how would I tell the Registration it is registerable_type Spreader so it
can create a new registration form that loads in the fields I need for
the Spreader as well as the fields for all registrations.  I am getting
ahead of myself."


Did you work out a solution for this?  here is the issue I am having :



I have created a polymorphic association with Assets being the main 
class and article, image and offers being the sub classes.

I am trying to work out what is the best way to create individual asset 
types (article, image or offer) and pass this request through to the 
controller.  Do I specify the asset to be created in the request url 
e.g. new/image or do ?  What other methods can I use

At present I hard code the asset type in the new and create controller 
for Asset.

I am fine with editing or showing already created assets, as i can 
determine the asset type by querying the .class value of the asset.

Any help would be appreciated.

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