Multi Select with fields_for

I’m trying to get a multi select box working with fields_for but am having a few problems.

My view has …

<% form_for(@teacher) do |f| %>
<%= f.label :surname %>
<%= f.text_field :surname %>

<% f.fields_for :qualifications do |qual_form| %>
Qualifications: <%= qual_form.select( :id, @qualifications) %>
<% end %>

<%= f.submit %>
<% end %>

Which produces (for the fields_for) …

<select id="teacher_qualifications_attributes_0_id" name="teacher[qualifications_attributes][0][id]">

  <option value="5">Licence</option>
  <option value="1">NPLQ</option>

  <option value="3">Teacher's Level One</option>
  <option value="4">Teacher's Level Two</option>

  <option value="2">Teacher's Rescue Test</option>
</select>

and on submission I have …

“teacher”=>{“qualifications_attributes”=>{“0”=>{“id”=>“2”}}, “surname”=>""}}

but what I think I really want is a multi select box with the submitted data looking something like … “teacher”=>{“qualifications_attributes”=>{“id”=>[“2”,“3”,“4”]}}, “surname”=>""}}

Could somebody putme out of my misery, please?

TIA