title: Can't save collection_select to object
body: <p>I'm going crazy here.</p>
  
  <p>I have:</p>
  
  <pre><code>class Course
     belongs_to :commune
  end
  </code></pre>
  
  <p>and</p>
  
  <pre><code>class Commune
     has_many :courses
  end
  </code></pre>
  
  <p>In the new view for Course I have a drop down where the user selects commune:</p>
  
  <pre><code>f.collection_select(:commune, get_commune_list, :id, :commune, { :prompt =&gt; true })
  </code></pre>
  
  <p>(The get_commune_list is a helper method that returns a list of Commune objects)</p>
  
  <p>But when I try to save it I either get this error:</p>
  
  <blockquote>
    <p>Commune(#2176182100) expected, got String(#2148246520)</p>
  </blockquote>
  
  <p>or the Commune isn't saved on the Course object at all.</p>
  
  <p>The parameters look like this:</p>
  
  <pre><code>{"course"=&gt;{"price"=&gt;"6000",
   "title"=&gt;"Some title",
   "commune"=&gt;"10",
   ...
  }
  </code></pre>
  
  <p>I just can'† figure out why this won't work!</p>