title: Why do I get SQLAlchemy nested rollback error?
body: <p>I got an error as follows in my code of python (which collects twitter statuses and store in database).</p>
  
  <pre><code>sqlalchemy.exc.InvalidRequestError: This Session's transaction has been rolled back by a nested rollback() call.  To begin a new transaction, issue Session.rollback() first.
  </code></pre>
  
  <p>I want to know what is the problem, why does it occur, and how can I solve it.</p>
  
  <p>I have no idea about <code>nested rollback</code>. Is there any simple example which occurs <code>nested rollback</code>?</p>