Rails form_for multiple collection_select，表单失败时未选择值

Question

I'm not sure why, but my form is not showing the options selected on submit, even though the params hash shows that the information is being returned to the page.

Collection select code:

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

Which renders:

<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 returned after form validation error

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

Update

I have also tried:

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

and:

<%= select_tag 'post_topic_ids', options_from_collection_for_select(PostTopic.all, "id", "name"), multiple: true %>

Which renders:

<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

you need to specify which element is selected a third parameter

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

look at http://apidock.com/rails/ActionView/Helpers/FormOptionsHelper/options_for_select for some examples.