title: custom field in rails form
body: <p>In my /app/views/institution/_form.html.erb I have </p>
  
  <pre><code>&lt;%= f.textfield :auto_complete_list %&gt;
  </code></pre>
  
  <p>Which gets its data from /app/models/institution.rb</p>
  
  <pre><code>  def auto_complete_list
      return self.county.city.name + ' '+ self.county.name
    end
  </code></pre>
  
  <p>But I don't want this to be submitted when the button is pressed.
  My current solution is to delete it from params[:institution]
  Is there a cleaner approach to removing read-only attributes from submit parameters?</p>