title: How to change database_url on heroku?
body: <p>I am trying to share a database between two apps on <code>Heroku</code> according to <a href="https://stackoverflow.com/questions/5981508/share-database-between-2-apps-in-heroku">Share database between 2 apps in Heroku</a> but setting up database_url on the second app gives an error:</p>
  
  <pre><code>$ heroku config:add DATABASE_URL=postgres://...
  Setting config vars and restarting pacific-headland-1960... failed  
   !    Cannot destroy last attachment to billing app for resource loving-subtly-5807
  </code></pre>