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copied!<p>To model the relationships you described you would do:</p> <pre><code>models/category.rb class Category < ActiveRecord::Base has_many :products has_many :subcategories, :class_name => "Category", :foreign_key => :parent_id end models/product.rb class Product < ActiveRecord::Base belongs_to :product has_many :features, :through => :product_features has_many :product_features end models/feature.rb class Feature < ActiveRecord::Base has_many :product_features has_many :products, :through => :product_features end models/productfeature.rb class ProductFeature < ActiveRecord::Base belongs_to :product belongs_to :feature end </code></pre> <p>Given this structure then you have the join modelled as a Many-to-Many relation. This is useful since the HABTM style of join is going away in Rails 3.1</p> <p>To get the information, I often use the console <strong>rails console</strong> for testing and this would allow you do do</p> <pre><code>@category = Category.first #get the first category @category.subcategories #returns an array of categories </code></pre> <p>The traversal of the links is via the relations that you setup in the models, with the intention that its <strong>readable</strong>, in the context of using sensible names. The self-joins, as per your question, is also covered in <a href="http://guides.rubyonrails.org/association_basics.html#self-joins" rel="nofollow">Rails Guides: Associations</a> with a good example. The rest of this guide also details the other relationships.</p> <p>One other thing to remember is to create your migrations so that the join table is created with the id's which are the foreign keys.</p>

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