title: Display a checkbox list instead of multiple select
body: <p>I have a model <code>MyModel</code> with a serialized attribute <code>a</code>, describing an array of symbols.</p>
  
  <p>This code works :</p>
  
  <pre><code>&lt;% form_for @my_model do |f| %&gt;
    &lt;%= f.select :a, MyModel::AS, :multiple =&gt; true) %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>The parameters are correct :</p>
  
  <pre><code>{ :my_model =&gt; { :a =&gt; [:a_value1, :a_value2] } }
  </code></pre>
  
  <p>I want to transform this multiple select into a set of checkboxes, like this :</p>
  
  <pre><code>&lt;% form_for @my_model do |f| %&gt;
    &lt;% MyModel::AS.each do |a_value|
      &lt;%= f.check_box(:a_value) %&gt;
    &lt;% end %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>It works too, but the parameters are not the same at all :</p>
  
  <pre><code>{ :my_model =&gt; { :a_value1 =&gt; 1, :a_value2 =&gt; 1 } }
  </code></pre>
  
  <p>I think of 2 solutions to return to the first solution...</p>
  
  <ul>
  <li>Transform my <code>check_box</code> into <code>check_box_tag</code>, replace multiple select, and add some javascript to 'check' select values when user clic on check_box_tags. Then, the parameter will be the same directly in the controller.</li>
  <li>Add a litte code into the controller for 'adapting' my params.</li>
  </ul>
  
  <p>What solution is the less ugly ? Or is there any other one ?</p>