Rails 3 has_many :through accessing attributes - ruby-on-rails-3

Question

I am working with a has_many through for the first time, and despite a lot of reading here and in the guide I am not understanding the correct way to access attributes on the through table. My tables are the same as this example from another post.
class Product < ActiveRecord::Base
has_many :collaborators
has_many :users, :through => :collaborators
end
class User < ActiveRecord::Base
has_many :collaborators
has_many :products, :through => :collaborators
end
class Collaborator < ActiveRecord::Base
belongs_to :product
belongs_to :user
end
Assuming that the collaborators table has additional attributes, say hours_spent, what is the correct way to find the hours_spent from the collaborator table for a particular user and product?
When I have found my users via the product, and am iterating over them as in
#product.users.each do |user|
This seems to work
user.collaborator[0].hours_spent
I get the correct value, but since there should only be one collaborator record for each User/Product pair, the index is throwing me off, making me think I’m doing something wrong.
Thank you for reading!
EDIT
Perhaps I am not getting the has_many through concept. Maybe a MySQL example would help.
What I was thinking is that if I did
SELECT * FROM collaborators where user_id = 1;
I would expect a set (zero or more) as the result. Similarly
SELECT * FROM collaborators where product_id = 1;
would also give me a set, but
SELECT * FROM collaborators where user_id = 1 and product_id = 1;
would give at most 1 row.
If I am understanding properly, all 3 queries return a set. So I guess I need some kind of uniqueness constraint, but that would have to be a compound key of sorts, on both of the belongs to keys. Is that even possible? Is there a structure that better models this?
Thanks so much for the quick and helpful responses!

Answer 1

There may be a single database row per pair, but when considering a single user, that user can be associated to many products, so a user can have many rows in the collaborators table. Similarly, when considering a single product, that product can be associated to many users, so a product can have many rows in the collaborators table.
Also, instead of using user.collaborators[0].hours_spent, use user.collaborators.first.try(:hours_spent) (which may return null), if you only want the first collaborator's hours spent.
If a single user can only have one single product and a single product can only have a single user, then switch the has_many's to has_one's for everything.
Update: The preceding is the answer to the original question which has since been clarified via comments. See comments for detail and see comments on other answer by Peter.

Answer 2

Perhaps you should use has_and_belongs_to_many. If your Collaborator is used only to make link between User and Product without having more fields.
class Product < ActiveRecord::Base
has_and_belongs_to_many :users
end
class User < ActiveRecord::Base
has_and_belongs_to_many :products
end
The beetween migration would be:
class CreateUsersProducts < ActiveRecord::Migration
def change
create_table "users_products", :id => false do |t|
t.integer :user_id
t.integer :product_id
end
end
end

Answer 3

After implementing this, what I found was that I think I had the correct relationships setup, I had to use the has_many :though as users could have many products, and it needed to be :through because there are additional attributes on the collaborator table. The sticking point was how to get there to only be a single Collaborator record for each user/product pair, and then how do I guarantee I got it. And to this point the answer I've found is it has to be done in code.
To make sure there is only a single record for each pair, I used
class Collaborator < ActiveRecord::Base
validates :product_id, :presence => true, :uniqueness => {:scope => [:user_id], :message => "This is a duplicate join"}
And then to make doubly sure I'm finding the right record, I have a scope
scope :collaboration_instance, lambda {|p_id, u_id| where("collaborations.product_id = ? && collaborations.user_id = ?", p_id, u_id)}
If someone has a more elegant solution, or just wants to improve this one, please post and I will change yours to the selected answer.