title: Integer field not autoincrementing in SQLAlchemy
body: <p>I have a Flask-SQLAlchemy model with an <code>Integer</code> field that I'd like to autoincrement. It's not a primary key; it's a surrogate ID. The model looks like:</p>
  
  <pre><code>class StreetSegment(db.Model):
      id = db.Column(db.Integer, autoincrement=True)
      seg_id = db.Column(db.Integer, primary_key=True)
  </code></pre>
  
  <p>When I create the table in my Postgres database, the <code>id</code> field is created as a plain integer. If I insert rows without specifying a value for <code>id</code>, it doesn't get populated. Is there some way I can force SQLAlchemy to use <code>SERIAL</code> even if it isn't the primary key?</p>