Problem with join table in rails

I have a join model that connects Authors to Products. It's called contracts. I want to create the contract as soon as the product is created, so in my product model I have:

     after_save :create_contract

     def create_contract
        contract = Contract.new(
          :product_id => self.id,
          :author_id => @author_id
        )
        contract.save   
     end

It seemed simple enough to me, but :author_id always comes up nil when it's ready to go into the database. I've tried a couple different ways of setting it, nothing seems to work. I'm guessing it's related to how I'm submitting it with the products form, which looks like this:

  <div class="field">
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :handle %><br />
    <%= f.text_field :handle %>
  </div>
  <div class="field">
    <%= f.label :description %><br />
    <%= f.text_area :description %>
  </div>
  <div class="field">
    <%= f.label :keywords %><br />
    <%= f.text_field :keywords %>
  </div>

    <div>
        <%= collection_select( "contract", "author_id", @authors, "id", "full_name") %>
    </div>

And in the controller:

  def create
    @author_id = params[:contract][:author_id]
    @product = Product.new(params[:product])
    ...
  end

Here's what I'm seeing in the log.

Parameters: {"utf8"=>"✓", "authenticity_token"=>"...", "product"=>{"title"=>"", "handle"=>"", "description"=>"", "keywords"=>""}, "contract"=>{"author_id"=>"1"}, "commit"=>"Create Product"}
  SQL (1.1ms)  INSERT INTO "products" ("created_at", "description", "handle", "keywords", "title", "updated_at") VALUES (?, ?, ?, ?, ?, ?)  [["created_at", Mon, 08 Aug 2011 04:37:09 UTC +00:00], ["description", ""], ["handle", ""], ["keywords", ""], ["title", ""], ["updated_at", Mon, 08 Aug 2011 04:37:09 UTC +00:00]]
  SQL (0.7ms)  INSERT INTO "contracts" ("author_id", "created_at", "product_id", "updated_at") VALUES (?, ?, ?, ?)  [["author_id", nil], ["created_at", Mon, 08 Aug 2011 04:37:09 UTC +00:00], ["product_id", 5], ["updated_at", Mon, 08 Aug 2011 04:37:09 UTC +00:00]]

Any ideas on where the problem is?

class Product < ActiveRecord::Base
  has_many :authors, :through => :contracts

and

class Author < ActiveRecord::Base
  has_many :product开发者_如何学JAVAs, :through => :contracts

and

class Contract < ActiveRecord::Base
  belongs_to :author
  belongs_to :product
end

So, jimworm's nested_attributes works, with two changes:

<%= f.fields_for :contract do |c| %>
  <%= c.collection_select :author_id, Author.all, :id, :name %>
<% end %>

(assuming <%= form_for(@product) do |f| %>)

and then, in the product controller:

def new
    @product = Product.new
    contract = @product.contracts.build
 ...

Your @author_id in create_contract is the Product model, and therefore not in the same scope as your controller.

Try putting the following in your models:

class Product < ActiveRecord::Base
  has_one :contract, :dependent => :destroy
  has_one :author, :through => :contract
  accepts_nested_attributes_for :contract
end

class Contract < ActiveRecord::Base
  belongs_to :product
  belongs_to :author
end

class Author < ActiveRecord::Base
  has_many :contracts, :dependent => :destroy
  has_many :products, :through => :contracts
end

Then in your form:

...
<%= f.fields_for :contract do |c| %>
  <%= c.collection_select :author_id, Author.all, :id, :name %>
<% end %>
...

Try this and see how it goes.