Rails form_for multiple collection_select，未在表單失敗時選擇的值

Question

我不確定爲什麼，但我的表單不顯示提交時選擇的選項，即使params散列顯示信息正在返回到頁面。

款式選擇代碼：

<%= f.collection_select :post_topic_ids, PostTopic.all, :id, :name, {}, { multiple: true, class: 'form-control' } %> 

這使得：表單驗證錯誤後

<select multiple="multiple" class="form-control" name="post[post_topic_ids][]" id="post_post_topic_ids"> 
    <option value="1">Psychology</option> 
    <option value="2">Engineering</option> 
    <option value="3">Nanotechnology</option> 
</select> 

PARAMS返回

params = {"post"=>{"post_topic_ids"=>["", "1"]}} 

更新

我也曾嘗試：

<%= select_tag 'post_topic_ids', options_for_select(PostTopic.all.collect{ |p| [p.name, p.id] }), multiple: true %> 

和：

​​

這使得：

<select name="post_topic_ids[]" id="post_topic_ids" multiple="multiple"><option value="1">Psychology</option> 
<option value="2">Engineering</option> 
<option value="3">Nanotechnology</option></select> 

Answer 1

你需要指定哪個元素選擇了第三個參數

<%= select_tag 'post_topic_ids', options_for_select(PostTopic.all.collect{ |p| [p.name, p.id] }, --->selected_element<---), multiple: true %> 

一些例子請看http://apidock.com/rails/ActionView/Helpers/FormOptionsHelper/options_for_select。