Rails: Up and Running, 2nd Edition

Trees

Let’s think about the most complex relationship in Photo Share: nested categories. You could implement categories by adding belongs_to :category and has_many :categories to the Category class. The resulting code would be awkward because a category would have an attribute called category (for the parent) and another called categories for the children. parent and children attributes would be better, but you’d be forced to override Active Record naming conventions and to write much more code.

This arrangement is common enough that Active Record has the acts_as_tree relationship, shown in Figure 4-6. Just as you did with acts_as_list, you will need to install a plug-in.

$ script/plugin install acts_as_tree
+ ./README
+ ./Rakefile
+ ./init.rb
+ ./lib/active_record/acts/tree.rb
...

acts_as_tree requires a foreign key called parent_id by default. Recall that we added parent_id to photos as part of the initial photos create_categories migration in Chapter 2. If you choose to use the default foreign key, Active Record will find it. Otherwise, you’d have to use the :foreign_key option. The methods and attributes added through acts_as_tree will use the foreign key to organize your tree, adding parent and children attributes to your class so each instance becomes a node of a tree. The root of the tree has a parent id of nil.

Figure 4-6. The acts_as_tree relationship is recursive, with an entity ...