Products - Categories association. (edit) belongs_to, has_and_belongs_to_many ?

Question

I created a joined table for the purpose. Here's the code:

class CreateCategoriesProductsJoin < ActiveRecord::Migration
  def self.up
    create_table 'categories_products', :id => false do |t|
      t.column 'category_id', :integer
      t.column 'product_id', :integer
    end
  end

  def self.down
    drop_table 'categories_products'
  end
end

The Product model:

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
  attr_accessor :new_category_name
  before_save :create_category_from_name

  def create_category_from_name
    create_category(:name => new_category_name) unless new_category_name.blank?
  end
end

And the category model:

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

So far so good. The problem is with the form.html.erb. code here:

<p>
      <label for="product_category_id">Category:</label><br />
      <%= f.collection_select :category_id, Category.find(:all), :id, :name, :prompt => "Select a Category" %>
      or create one:
      <%= f.text_field :new_category_name %>
</p>

I try to load the categories in a selection box but I get a message "undefined method `category_id' for #".

I don't understand why? I also tried with has_many :through but the same thing happened.

I should be able to draw the categories out with that way since the are associated but I'm not. Any ideas?

I tried the screancast from railscast.com but no luck. Do you have in mind a full tutorial/example with product category, subcategory associations? I haven't found a descend one yet ( except for the railscast but that doesn't work for me ).

Thank you very much for your time and thank you for the trouble it took you to respond ( if you did ).

Edit: I found the problem it was in the product model. Here's the fix:

class Product < ActiveRecord::Base
  belongs_to :category

  def category_name
    category.name if category
  end

  def category_name=(name)
    self.category = Category.find_by_name(name) unless name.blank?
  end

end

Edit 2: I also have another problem with the categories.

Categories name doesn't appear in the view. I call the category_name function but I get nothing in return. Like this (show.html.erb):

<p>
      <label for="product_category_id">Category:</label><br />
      <%= f.collection_select :category_name, Category.find(:all), :id, :name, :prompt => "Select a Category" %>
      or create one:
      <%= f.text_field :new_category_name %>
</p>

Or (show.html.erb):

<td><%= product.category_name %></td>

I tried to change to has_and_belongs_to_many :categories and at least I got back "Category" as a name which is weird... because I have 4 category.

A products belongs only to one category.

PS: Again, thank you for your time.

Answer 1

<%= f.collection_select :category_id ... %>

That line is looking for .category_id on the form object, which has accessors dependent on the object the form was created for (I'm assuming it is a product object). The product object has attributes for the has_and_belongs_to_many relationship with category, but it is not a single category id. The methods available are listed here. I'm not certain because I don't ever use has_and_belongs_to_many, but I think you'll want

<%= f.collection_select :categories_singular_ids ... %>

The collection_select will need to be a multi-select, so for your html you'll want :multiple => true in the last options hash.