Nested Set Error undefined method `self_and_descendants' for #<ActiveRecord::Relation:0x52c4a30>

问题

I am using the Nested_Set gem in my code to sort Categories, Subcategories, and Products. I am trying to limit the depth/level of my nested set to no deeper then 2. Currently I am getting the error

Nested Set Error undefined method `self_and_descendants' for #<ActiveRecord::Relation:0x52c4a30> 

I am trying to create a restraunt menu type style, and am going to attempt to make it drag and drop sortable.

Here is my code: Can someone browse it and help me understand this error? Thanks Category.rb

class Category < ActiveRecord::Base
  acts_as_nested_set
  acts_as_list :scope => :parent_id
  has_many :products
  scope :category, where("parent_id IS NULL")
  scope :subcategories, where("parent_id IS NOT NULL")
  scope :with_depth_below, lambda { |level| where(self.arel_table[:depth].lt(level)) }
end

categories_controller

class CategoriesController < ApplicationController
  def new
    @category = params[:id] ? Category.find(params[:id]).children.new : Category.new
    @count = Category.count
  end

  def new_subcategory
    @category = params[:id] ? Category.find(params[:id]).children.new : Category.new
    @category_2_deep = Category.with_depth_below(2)
  end

  def create
    @category = params[:id] ? Category.find(params[:id]).children.new(params[:category]) : Category.new(params[:category])
    if @category.save
      redirect_to products_path, :notice => "Category created! Woo Hoo!"
    else
      render "new"
    end
  end

  def edit
    @category = Category.find(params[:id]) 
  end

  def edit_subcategory
    @category = Category.find(params[:id]) 
    @category_2deep = Category.with_depth_below(2).arrange
  end

  def destroy
    @category = Category.find(params[:id])
    @category.destroy
    flash[:notice] = "Category has been obliterated!"
    redirect_to products_path
  end

  def update
    @category = Category.find(params[:id])
    if @category.update_attributes(params[:category])
      flash[:notice] = "Changed it for ya!"
      redirect_to products_path
    else 
      flash[:alert] = "Category has not been updated."
      render :action => "edit"
    end
  end

  def show
    @category = Category.find(params[:id])
  end

  def index
  end

  def sort
    params[:category].each_with_index do |id, index|
      Category.update_all({position: index+1}, {id: id})
    end
  end
end

Routes.rb

Jensenlocksmithing::Application.routes.draw do
  get "log_out" => "sessions#destroy", as: "log_out"
  get "log_in" => "sessions#new", as: "log_in"
  get "site/home"
  get "site/about_us"
  get "site/faq"
  get "site/discounts"
  get "site/services"
  get "site/contact_us"
  get "site/admin"
  get "site/posts"
  get "categories/new_subcategory"
  get "categories/edit_subcategory"

  resources :users
  resources :sessions
  resources :coupons
  resources :monthly_posts
  resources :categories do
    collection { post :sort }
    resources :children, :controller => :categories, :only => [:index, :new, :create, :new_subcategory]
  end
  resources :subcategories
  resources :products
  resources :reviews
  resources :faqs do
    collection { post :sort }
  end 

  root to: 'site#home'
end

categories/form.html.erb

<%= form_for(@category) do |f| %>

<p>
<%= f.label(:name) %>
<%= f.text_field :name %>
</p>
<p>
<%= f.label(:parent_id) %>
<%= f.select :parent_id, nested_set_options(@category_2_deep, @category) {|i, level| "# {'-' * level if level < 1 } #{i.name if level < 1 }" } %>

</p>
<p>
<%= f.label(:position) %>
<%= f.select :position, 1..category_count %>
</p>
<p>
  <%= f.submit("Submit") %>
</p>
<% end %>

回答1:

It looks like nested_set is looking for an Array not just an array-like collection - see line 32 of the source: https://github.com/skyeagle/nested_set/blob/21a009aec86911f5581147dd22de3c5d086355bb/lib/nested_set/helper.rb#L32

...hence it's getting that ActiveRecord::Relation, wrapping it in an [array] (line 35) and then trying to iterate and blowing up.

Easy fix: call to_a on the collection first - in your controller:

@category_2_deep = Category.with_depth_below(2).to_a

Better fix: submit a patch to the maintainer that's a bit more Ruby-like and looks for it to behave like an Array, but not necessarily be one.

来源：https://stackoverflow.com/questions/9459662/nested-set-error-undefined-method-self-and-descendants-for-activerecordrel