title: limit choices with dropdown in flask-admin
body: <p>My SQLAlchemy model has a <code>String</code> field that I would like to limit to a few choices. </p>
  
  <p>I am wondering how I would be able to create a dropdown for this field in the Flask-Admin interface that would ensure that the db is only populated with one of my choices. If I let the user manually enter these fields, they may spell them incorrectly, etc.</p>