sql - How to render all records from a nested set into a real html tree

Question

I'm using the awesome_nested_set plugin in my Rails project. I have two models that look like this (simplified):

class Customer < ActiveRecord::Base
  has_many :categories
end

class Category < ActiveRecord::Base
  belongs_to :customer

  # Columns in the categories table: lft, rgt and parent_id
  acts_as_nested_set :scope => :customer_id

  validates_presence_of :name
  # Further validations...
end

The tree in the database is constructed as expected. All the values of parent_id, lft and rgt are correct. The tree has multiple root nodes (which is of course allowed in awesome_nested_set).

Now, I want to render all categories of a given customer in a correctly sorted tree like structure: for example nested <ul> tags. This wouldn't be too difficult but I need it to be efficient (the less sql queries the better).

Update: Figured out that it is possible to calculate the number of children for any given Node in the tree without further SQL queries: number_of_children = (node.rgt - node.lft - 1)/2. This doesn't solve the problem but it may prove to be helpful.

Answer 1

It would be nice if nested sets had better features out of the box wouldn't it.

The trick as you have discovered is to build the tree from a flat set:

• start with a set of all node sorted by lft
• the first node is a root add it as the root of the tree move to next node
• if it is a child of the previous node (lft between prev.lft and prev.rht) add a child to the tree and move forward one node
• otherwise move up the tree one level and repeat test

see below:

def tree_from_set(set) #set must be in order
  buf = START_TAG(set[0])
  stack = []
  stack.push set[0]
  set[1..-1].each do |node|
    if stack.last.lft < node.lft < stack.last.rgt
      if node.leaf? #(node.rgt - node.lft == 1)
        buf << NODE_TAG(node)
      else
        buf << START_TAG(node)
        stack.push(node)
      end
    else#
      buf << END_TAG
      stack.pop
      retry
    end
  end
  buf <<END_TAG
end

def START_TAG(node) #for example
  "<li><p>#{node.name}</p><ul>"
end

def NODE_TAG(node)
  "<li><p>#{node.name}</p></li>"
end

def END_TAG
  "</li></ul>"
end