awesome nested set, .rebuild! method fully commented (WIP)

awesome_nested_set.rb

# Rebuilds the left & rights if unset or invalid.  Also very useful for converting from acts_as_tree.
def rebuild!
  # Don't rebuild a valid tree.
  return true if valid?

  # defines the method _scope_ to return _nil_
  scope = lambda{|node|}
  
  # if the _:scope_ option was set, we must use it
  if acts_as_nested_set_options[:scope]
    
    # redefine the _scope_ method
    scope = lambda{|node|
      
      # scope_column_names is an Array of scopes
      scope_column_names.inject(""){|str, column_name|
        
        # we go through all scopes and add them to a string that will be used on _:conditions_
        # example: if _:scope_ was defined as _:company_, _:company_id_ will be used and this will return the following string:
        # "AND company_id = 5"
        # (ignoring db quoting stuff and assuming the passed node's company_id is _5_)
        str << "AND #{connection.quote_column_name(column_name)} = #{connection.quote(node.send(column_name.to_sym))} "
      }
    }
  end
  indices = {}

  # define the +set_left_and_rights+ method (returns true if the +node.save!+ succeeds)
  set_left_and_rights = lambda do |node|
    
    # set left
    node[left_column_name] = indices[scope.call(node)] += 1
    
    # find sub-nodes and set left/right for them
    find(
      :all, 
      :conditions => ["#{quoted_parent_column_name} = ? #{scope.call(node)}", node], 
      :order => "#{quoted_left_column_name}, #{quoted_right_column_name}, id"
    ).each{|n| set_left_and_rights.call(n) }
    
    # set right
    node[right_column_name] = indices[scope.call(node)] += 1
    
    # save node
    node.save!
  end

  # Find root node(s)
  root_nodes = find(
    :all, 
    :conditions => "#{quoted_parent_column_name} IS NULL", 
    :order => "#{quoted_left_column_name}, #{quoted_right_column_name}, id"
  ).each do |root_node|
    # setup index for this scope
    indices[scope.call(root_node)] ||= 0
    set_left_and_rights.call(root_node)
  end
end