title: rails: collection_select for current_user
body: <p>I have a Box which has many Items which belongs to a Category.
  When the Box is created it has the user_id.
  Items has a category_id.</p>
  
  <p>I've seen that if you have a user_id in the table concerned it works okay:</p>
  
  <pre><code>&lt;%= f.collection_select :category_id, Category.where(:user_id =&gt; current_user.id), :id , :name %&gt;
  </code></pre>
  
  <p>But how can I get a Category collection_select from what the current_user has entered - do I have to add a user_id to Categories or can I join my tables somehow?</p>