Rails4 別modelのカラム情報を表示する

  def list_params
    params.require(:category_id)
  end

 def list
    selected_category = Category.where(parent_id: list_params)#list_paramsで拾ったcategory_idが親カテゴリになっている子カテゴリを集める

    childs = selected_category.sort{|a, b| a.code <=> b.code}
    data = []
    childs.each do |child|
      data << {id: child.id, name: child.name}
    end
    respond_to do |format|
      format.json { render json: data }
    end
  end


  def category_select
    root = Category.find(1)
    @categories = root.childs.sort{|a, b| a.code <=> b.code}
    @childs = @categories[0].childs.sort{|a, b| a.code <=> b.code}
  end

    <!-- アイテム情報 -->
    <div class="panel panel-default">
      <div class="panel-heading">
        <h3 class="panel-title"> アイテム情報</h3>
      </div>
      <ul class="list-group list-group-flush text-left">

        <li class="list-group-item">
          <%= @item.content %>
        </li>




        <li class="list-group-item">
          <strong>カテゴリ:</strong>
          <%= @item.category_id %>
        </li>
        <li class="list-group-item">
          <strong>subカテゴリ:</strong>
          <%= @item.sub_category_id %>
        </li>

        <li class="list-group-item">
          <strong>コンディション:</strong>
          <%= @item.condition %>
        </li>

        <li class="list-group-item">
          <strong>発送元:</strong>
          <%= @item.area %>
        </li>

        <li class="list-group-item">
          <strong>執行日:</strong>
          <%= @item.limit_day %>
        </li>
      </ul>
    </div>

$(document).on('change', '#category_select', function(evt) { 
                    $.ajax({ 
                        type: 'GET', 
                        url: 'categoreis/list', 
                        dataType: 'json', 
                        data: { 
                            category_id: $("#category_select option:selected").val() 
                        }, 
                        error: function(jqXHR, textStatus, errorThrown) { 
                            console.log("AJAX Error: " + textStatus); 
                        }, 
                        success: function(data, textStatus, jqXHR) { 
                            $("#child_select").empty(); 
                            if (data.length == 0) { 
                                $("#child_select").hide(); 
                            } else { 
                                $("#child_select").show(); 
                            } 
                            $.each(data, function(idx, child) { 
                                // console.log("" + item.code + ":" + item.name); 
                                $("#child_select").append('<option value="' + child.id + '">' + child.name + '</option>'); 
                            }); 
                            console.log("Dynamic country select OK!"); 
                        } 
                    }); 
                });