Jun 25, 2008

named_scope with acts_as_tree

I fairly often use the acts_as_tree plugin in my applications. While acts_as_nested_set (and superior variants..) is more powerful, often times a simple two-level deep hierarchy is all I need and acts_as_tree is simple. I've found the new named_scope functionality in Rails 2.1 to be very helpful when dealing with tree data structures.

Firstly, it's somewhat rare that I have one single root node in the tree structure (which is apparently how it's meant to be used). Instead I'll have multiple "parent" nodes designated with a NULL parent_id and children beneath. In the past I've always done something like: find(:all, :conditions => {:parent_id => nil}) to grab the top entries. Instead with nested set you can do this:

named_scope :top, :conditions => {:parent_id => nil}
                # Then:

Another common task when dealing with hierarchical categories is to query on the base object (products for example) for members that are "part of" that category. Specifically, part of in a 2-level heirarchy simply means "where id = ? or parent_id = ?" on the joined categories table. Because this involves a join it was somewhat clunky to do before. Now with nested set, on the product model I can declare this:

named_scope :in_category, lambda {|c| {:include => [:category], :conditions => ["categories.id = ? OR categories.parent_id = ?", c, c]} }
                # Then:

I would also highly encourage you all to check out RailsCasts Episode 112 "Anonymous Scopes" by Ryan Bates where he outlines a pattern for handling conditions elegantly with searches using named_scopes in Rails 2.1, something I've always found to be lacking from Rails core and always resorted to using plugins like criteria_query.

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