title: alembic util command error can't find identifier
body: <p>I'm trying to use alembic to handle local migrations on my project. It worked the first time, but then I needed to delete the folder and restart.(don't ask why, I just had to) I'm following <a href="https://realpython.com/blog/python/flask-by-example-part-2-postgres-sqlalchemy-and-alembic/" rel="noreferrer">this tutorial</a> and I run the command</p>
  
  <pre><code>python manage.py db init
  </code></pre>
  
  <p>And it was ok. But when I try to run</p>
  
  <pre><code>python manage.py db migrate
  </code></pre>
  
  <p>I'm getting this error:</p>
  
  <pre><code>alembic.util.CommandError: Can't locate revision identified by '31b8ab83c7d'
  </code></pre>
  
  <p>Now, it seems that alembic is looking for a revision that doesn't exists anymore. There is anyway to make alembic forget that file? Or like restart the comparison from None to -> auto-generated again?  </p>