title: rails - what exactly does hidden_field and hidden_field_tag do?
body: <p>I read through the techy definition of <code>hidden_fields</code>, but am not sure what it really does.  My understanding is that it allows you to pass in an attribute for certain parameters.  For example, if you have a rich join model, you can use the <code>hidden_field</code> to assign the user_id to the join model attribute for user. Is that correct?</p>
  
  <p>If so, would it be better to do it in the form or the controller?  </p>