{"id":153,"date":"2010-04-14T13:20:46","date_gmt":"2010-04-14T13:20:46","guid":{"rendered":"http:\/\/nicholshayes.myzen.co.uk\/blog\/?p=153"},"modified":"2010-05-18T14:45:17","modified_gmt":"2010-05-18T14:45:17","slug":"self-referencing-objects","status":"publish","type":"post","link":"http:\/\/nicholshayes.co.uk\/blog\/?p=153","title":{"rendered":"Self referencing objects"},"content":{"rendered":"

My application has products, and each product can have components which are themselves products. I achieve this via a pair of self-referencing has_and_belongs_to_many relationships.<\/p>\n

  has_and_belongs_to_many :components<\/span>,
\n    :class_name<\/span> =><\/span> 'Product'<\/span>,
\n    :join_table<\/span> =><\/span> "product_grouping"<\/span>,
\n    :association_foreign_key<\/span> =><\/span> "component_product_id"<\/span>,
\n    :foreign_key<\/span> =><\/span> "group_product_id"<\/span>
\n
\n  has_and_belongs_to_many :groups<\/span>,
\n    :class_name<\/span> =><\/span> 'Product'<\/span>,
\n    :join_table<\/span> =><\/span> "product_grouping"<\/span>,
\n    :association_foreign_key<\/span> =><\/span> "group_product_id"<\/span>,
\n    :foreign_key<\/span> =><\/span> "component_product_id"<\/span><\/div><\/div>\n

Note the presence of the join table product_grouping which has two fields: group_product_id and component_product_id<\/p>\n

I wanted to be able to collect together all components within a product hierarchy. So if my products are car, engine and piston: piston is a component of engine, which is a component of car. However, car.components would only return engine. I needed a method that would traverse the whole component hierarchy.<\/p>\n

I achieved this by using a method that referenced itself within a collect:<\/p>\n

  def<\/span> all_components
\n    components.collect<\/span>{<\/span>|<\/span>c|<\/span> [<\/span>c]<\/span> <<<\/span> c.all_components<\/span>}<\/span>.flatten<\/span>.uniq<\/span>
\n  end<\/span><\/div><\/div>\n

I’ve seen this technique used before, but always had a little difficulty getting my head around it. I found that the key to getting it working was to make sure that the method always returned an array or nil.<\/p>\n

Now:<\/p>\n

  car.components<\/span> -><\/span> [<\/span>engine]<\/span>
\n  car.all_components<\/span> -><\/span> [<\/span>engine, piston]<\/span><\/div><\/div>\n

What effectively is happening with car.all_components is this:<\/p>\n