Multiple relationships between two models

Hello all. I'm having trouble understanding how to set up multiple
relationships between two models.

I have a database for an art gallery. It will be used to track people
(artists, buyers, donors, press contacts, etc) and artworks. Thus, I
have three main tables: "contacts," "roles," and "items," plus a join
table "contact_roles." A contact person can have multiple roles, i.e.
an artist or press contact could also buy an artwork, or a donor could
also be a volunteer.

Here are the tables and the classes:

create_table :contacts do |t|
  t.column :first_name, :string
  t.column :last_name, :string
end

create_table :roles do |t|
  t.column :name, :string
end

create_table :contact_roles do |t|
  t.column :contact_id, :string
  t.column :role_id, :string
end

create_table :items do |t|
  t.column :name, :string
  t.column :dimensions, :string
  t.column :medium, :string
  t.column :artist_id, :string
  t.column :buyer_id, :string
end

class Contact < ActiveRecord::Base
  has_and_belongs_to_many :roles

  def full_name_last
  last_name + ", " + first_name
  end
end

class Artist < Contact
  has_many :items
end

class Buyer < Contact
  has_many :items
end

class Role < ActiveRecord::Base
  has_and_belongs_to_many :contacts
end

class Item < ActiveRecord::Base
  belongs_to :artists
  belongs_to :buyers
end

My main question is: should I be using polymorphic associations to link
the contacts and the items? I don't quite understand polymorphic
associations - any suggestions on web pages with good explanations for
an object-oriented newbie?

If I don't need to use polymorphism, then do I need to use parameters
like :foreign_key on the has_many and belongs_to due to the fact that
artist/buyer are not semantically identical to "contacts"?

Thanks
David

http://wiki.rubyonrails.org/rails/pages/UnderstandingPolymorphicAssociations

I used it for Ratings, for example. Pretend you have a blog, which has
Posts, Comments, and Pictures. You want all of them to have Rating,
so use polymorphic.

I don't know how this would apply here, though.

-Ryan

Hello Ryan, thanks for your reply. That was a helpful link, but I
don't understand what happens in their example if an Order and a User
have the same address. Do you wind up with two records in the
addresses table, both the same except for addressable_id and
addressable_type?

David

Correct. You get tso records
address="1210 Orange St", addressable_id="5", addressable_type="User"
address="1210 Orange St", addressable_id="7", addressable_type="Order"

Basically you'll probably want to keep them seperate too, because if
you change a User's address, why would you change the address of an
order that's already been shipped/processed?

-Ryan

Thanks again. I don't want to duplicate my item records, so I did
some more searching and found the correct model definitions for what I
need:

class Contact < ActiveRecord::Base
  has_many :artist_items, :class_name => "Item", :foreign_key =>
"artist_id"
  has_many :buyer_items, :class_name => "Item", :foreign_key =>
"buyer_id"
  def full_name_last
    last_name + ", " + first_name
  end
end

class Item < ActiveRecord::Base
  belongs_to :artists, :class_name => "Contact", :foreign_key =>
"artist_id"
  belongs_to :buyers, :class_name => "Contact", :foreign_key =>
"buyer_id"
end