title: Form to select an instance
body: <p>I want my app to let the user choose between a few addresses or create one. The address is to be stored inside the contract object (Contract.adresse_id).</p>
  
  <p>The controller generating the page displaying the form generate a @user variable.
  The goal is to either pass the id of an existing address to the next controller, or to pass the params of a new adress (classic nested). This page only goal is to set the adress, no other fields of Contract should be modified.</p>
  
  <p>The model Contract has a address_id field. I want a radio button allowing to choose between :</p>
  
  <ul>
  <li><code>@user.address # it is an address id</code></li>
  <li><code>@user.secondary_adress # address id too</code></li>
  <li>and an empty form allowing to create your own address.(classic  nested)</li>
  </ul>
  
  <p>i guess it should look like :</p>
  
  <pre><code>&lt;%= form_for contract do |f|%&gt;        
  &lt;% f.label :address%&gt;   
  &lt;% end%&gt;
  </code></pre>
  
  <p>But then i do not know what to do. How can i do a form allowing to chose between 3 addresses where 2 already exists and the third is to be created ?</p>