How to Model Multiple Relationships in ActiveRecord?

Question

Hi,

I am trying to finish building a Rails "Home Inventory" app as an example to help me learn rails. Following gives a general overview of what I am trying to achieve:

The main purpose of this application is to show a page with detailed information.

So, http://localhost:3000/living-room-couch would display the information regarding the couch.

Each Item, can belong one( or has one?) of three categories:

1. Book
2. Furniture
3. Electronics.

Book has the following properties:
- isbn,
- pages,
- address,
- category

Furniture has following properties:
- color,
- price,
- address,
- category

Electronics has following properties:
- name,
- voltage,
- address,
- category.

--

Now on my View side, I have made 3 templates in rails, that contain elements suited to display an item belonging to one of the 3 categories. Template for Book shows isbn, and template for Electronics shows the voltage.

How do I model this in ActiveRecord? I will put it in English, maybe someone can help translate into Rails:

An Item, belongs_to or has_one category. Category can be one of the three: Book, Furniture, or Electronic.

I don't know how to do this. I know that each category like Book will be a model of its own, due to having different characteristics.

Do I need to have Category has a model too, because it would only consist of Book, or Furniture or Electronics. If I was to take the route of making Category a model of its own, how would I make it relate to a model such as Book.

---

--or

Would I just go this route (or join model perhaps):

class BookModel < ActiveRecord::Base
  has_many :categories
End

And then, select to which category belongs, based on the Model name.

I hope I put the question right, I am just so confused regarding this.

Thank You for your time.

Answer 1

I don't think you need the category model. You can just create the book, furniture and electronic models and one controller for each one.

Edit

No, there's no advantage. I just said to create different models/controllers because what you're modeling, in my opinion, are different things. But if you think they'll have a lot in common, I'd suggest for you to use single table inheritance. This way you'd have a single table for every item and can have only one controller, but each item would have one model.

Answer 2

Why not have an Item have the following relationships:

belongs_to :book
belongs_to :furniture
belongs_to :electronic

You can then only actually set on of these, and can do a test of

if item.book
#do things
end

for each item, if you care about whether it's a book, etc. You can even use validations to make sure it only ever belongs to one thing at a time.

Answer 3

the Item model could be declared as polymorphic (se here: http://guides.rails.info/association_basics.html#polymorphic-associations)

so you'll declare Item:

class Item < ActiveRecord::Base
  belongs_to :category, :polymorphic => true
end

then the 3 models that act as category (I'll show one, the others are same):

 class Employee < ActiveRecord::Base
   has_one :item, :as => :category
 end

 # and so on...

this way you can associate an Item to one of the 3 models (Book, Furniture and Electronics). each instance of these models will have an 'items' atribute, like this:

@book.items # returns all the items associated to a @book object

You can also use has_many association using a polymorphic model.

For the views, you can use Nested Form Objects (see here: http://guides.rails.info/form_helpers.html). basically, in each form, you should nest a form to create an Item object. to follow the Book example, you'll have something like this:

 <%= form_for :book, @book do |form| %>
   <%= form.text_field :isbn %>
   <!-- other fields -->
   <%= fields_for @book.item do |item_form| %>
      <%= item_form.text_field :name %>
   <% end %>
 <% end %>

hope this helped you ;)