Validating a node move in a nested set

Question

I am creating a Category model and using the awesome_nested_set plugin (a replacement for acts_as_nested_set) to deal with the hierarchy. With awesome_nested_set, objects are created, then saved, and then placed within the set. As well, lft, rgt and parent_id are attr_protected so they cannot be written to directly.

I am running into two situations when placing the node into the set that I want to be able to catch so that I notify the user (there might be more that I haven't thought of yet):

1. A node is attempted to be placed as its own child (self.id == self.parent_id)
2. A node is attempted to be moved beneath its own descendant (self.descendants.include? self.parent_id == true)

In both cases, the move will fail, but awesome_nested_set will only raise an ActiveRecord::ActiveRecordError exception, with a message that is not as descriptive as I'd like to be able to give the user.

awesome_nested_set has a number of node moving methods, which all call move_to(target, position) (where position is one of :root, :child, :left or :right and target is the related node for all positions but :root). The method fires a before_move callback, but doesn't provide a way that I can see to validate a move before it happens. In order to validate a move, I'd need access to the target and position, which the callback does not receive.

Does anyone know of either a way to validate a move in awesome_nested_set (either by having a way to pass target and position to the before_move callback of by another method), or another nested set plugin that will let me validate? I'd prefer not to fork or write my own plugin.

Answer 1

Here is the solution I came up with:

class Category < ActiveRecord::Base
  acts_as_nested_set :dependent => :destroy

  #=== Nested set methods ===

  def save_with_place_in_set(parent_id = nil)
    Category.transaction do
      return false if !save_without_place_in_set
      raise ActiveRecord::Rollback if !validate_move parent_id

      place_in_nested_set parent_id
      return true
    end

    return false
  end

  alias_method_chain :save, :place_in_set

  def validate_move(parent_id)
    raise ActiveRecord::RecordNotSaved, "record must be saved before moved into the nested set" if new_record?
    return true if parent_id.nil?

    parent_id = parent_id.to_i

    if self.id == parent_id
      @error = :cannot_be_child_of_self
    elsif !Category.all.map(&:id).include?(parent_id)
      @error = :given_parent_is_invalid
    elsif descendants.map(&:id).include? parent_id
      @error = :cannot_be_child_of_descendant
    end

    errors.add(:parent_id, @error) if @error
    return @error.nil?
  end

  def place_in_nested_set(parent_id)
    if parent_id.nil? || parent_id.blank?
      move_to_root
    else
      move_to_child_of parent_id
    end
    return true
  end
end

Now, in the controller, I just need to say @category.save(parent_id), where parent_id is nil or the ID of the parent, and the validation, node placement, and save is handled in the model.