Rails 階層カテゴリー 編集時の取得

#item.rb

class Item < ApplicationRecord
 belongs_to :category
end

#category.rb

class Category < ApplicationRecord
  has_many :items
  has_ancestry
end

#items_controller.rb

class ItemsController < ApplicationController
  before_action :set_item, only: [:show, :edit, :update, :destroy]
  before_action :set_parents, only: [:new, :create,:edit, :update]

  def index
  end

  def new
    @item = Item.new
  end

  def create
    @item = Item.new(item_params)
    if @item.save
      redirect_to root_path
    else
      render :new
    end
  end

  def edit
  end

  def update
    if @item.update(item_params)
      redirect_to item_path(@item.id)
    else
      render :edit
    end
  end

  def destroy
    item = Item.find(params[:id])
    item.destroy
    redirect_to root_path
  end


  def show
  end

  def get_category_children
    @category_children = Category.find("#{params[:parent_id]}").children
  end

  private

  def item_params
    params.require(:item).permit(:title, :category_id).merge(user_id: current_user.id)
  end

  def set_item
    @item = Item.find(params[:id])
  end

  def set_parents
    @parents = Category.where(ancestry: nil)
  end

end

#category.js

$(function(){
  function appendOption(category){
    var html = `<option value="${category.id}">${category.name}</option>`;
    return html;
  }
  function appendChildrenBox(insertHTML){
    var childSelectHtml = "";
    childSelectHtml = `<div class="category__child" id="children_wrapper">
                        <select id="child__category" name="item[category_id]" class="category_child">
                          <option value="">選択してください</option>
                          ${insertHTML}
                        </select>
                      </div>`;
    $('.append__category').append(childSelectHtml);
  }

  $('#item_category_id').on('change',function(){
    var parentId = document.getElementById('item_category_id').value;
    if (parentId != ""){
      $.ajax({
        url: '/items/get_category_children/',
        type: 'GET',
        data: { parent_id: parentId },
        dataType: 'json'
      })
      .done(function(children){
        $('#children_wrapper').remove();
        var insertHTML = '';
        children.forEach(function(child){
          insertHTML += appendOption(child);
        });
        appendChildrenBox(insertHTML);
        if (insertHTML == "") {
          $('#children_wrapper').remove();
        }
      })
      .fail(function(){
        alert('カテゴリー取得に失敗しました');
      })
    }else{
      $('#children_wrapper').remove();
    }
  });

});