title: Rails update join table attributes
body: <p>I have User, Attachment and Form models</p>
  
  <pre><code> form = @user.forms.where(attachment_id: attachment.id)
   form.update_attributes(status: "full")
  </code></pre>
  
  <p>when I try to update a join table attribute, it says</p>
  
  <pre><code>undefined method `update_attributes
  </code></pre>
  
  <p>in my models there are only necessary has_many or belongs to attributes. I don't understand why update_attributes does not work</p>