我有一个 form 表单，用的是 simple_form

<%= simple_form_for([:admin, @category], :wrapper => :inline) do |f| %>
  <% if @category.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@category.errors.count, "error") %> prohibited this category from being saved:</h2>
      <ul>
      <% @category.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

 <%= f.input :name %>
 <%= f.input :parent,  :collection => Category.all%>

  <div class="actions">
     <%= f.button :submit %>
  </div>
<% end %>

但事提交的时候

ActiveRecord::AssociationTypeMismatch in Admin::CategoriesController#create
Category(#70187922823040) expected, got String(#70187915399620)