title: ActiveStorage: Detach (instead of purge) attachment after owner record is destroyed?
body: <p>The Rails API documentation states <a href="https://api.rubyonrails.org/classes/ActiveStorage/Attached/Macros.html#method-i-has_many_attached" rel="noreferrer">the following</a> about <code>has_many_attached</code> and <code>has_one_attched</code> methods:</p>
  
  <blockquote>
    <p>If the :dependent option isn't set, all the attachments will be purged (i.e. destroyed) whenever the record is destroyed.</p>
  </blockquote>
  
  <p>Specifically, what value should I specify for this option?</p>