Polymorphic routes

child.rb

class Child < ActiveRecord::Base
  belongs_to :parent
  has_many :leaves, :as => :branch
  
  def route_parts 
    [parent, self]
  end
end

leaf.rb

class Leaf < ActiveRecord::Base
  belongs_to :branch, :polymorphic => true
end

parent.rb

class Parent < ActiveRecord::Base
  has_many :children
  has_many :leaves, :as => :branch
  
  def route_parts
    [self]
  end
end

routes.rb

FooBar::Application.routes.draw do
  resources :parents do
    resources :leaves

    resources :children do
      resources :leaves
    end
  end
end

shared_view.html.haml

= link_to 'New Leaf', url_for([:new] + record.route_parts + [:leaf])