belongs_to has_many assosiation help

I created a joined table for the models category and products (both created with scaffold). The Product model is this:

class Product < ActiveRecord::Base
  belongs_to :category 

  def category_id
    category.id if category
  end

  def category_id=(id)
    self.category = Category.find_by_id(id) unless id.blank?
  end
end

and the category model is this:

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

In the form.html.erb I create a dropbox with all the classes for the user to choose from:

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

Yet when I take a look at the show of the prod开发者_C百科uct:

<p>
  <b>Category:</b>
  <%= @product.category_id %>
</p>

or the list of the products (index.html.erb):

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

There's no category. Just blank. I don't get it. Is something wrong with the category_id method or the association?

Firstly, you don't need the explicit category_id and category_id= methods. ActiveRecord will handle those for you for a belongs_to association.

Secondly, there seems to be a mismatch between whether you want a has_and_belongs_to_many or a has_many/belongs_to association. If you have a join table then you have the former, in which case both sides of the association should be declared with has_and_belongs_to_many. If you are just using a category_id on the products table then the other end of your association on Category should be has_many :products.

With a join model:

class Categorization < ActiveRecord::Base
  belongs_to :category
  belongs_to :product
end

you would define in your Product class:

has_many :categorizations
has_many :categories, :through => :categorizations

Then, because your association is a 'many' association, you do not get a .category method on a product. You do however get a categories method (plus several more methods - look at the has_many documentation). If you name your collection_select category_ids then it should work as expected. You may also want to add the 'multiple' option to the select in order to choose more than one category.

Your association is obviously incorrect. As pointed out, the category has_many products. And in case you want to use a many-to-many relationship you're strongly advised to use the has_many :through relationship.