title: An attempt to mix objects belonging to different transactions
body: <p><code>pony.orm.core.TransactionError: An attempt to mix objects belonging to different transactions</code></p>
  
  <p>I know that this error is descriptive, I just can't figure out WHY its occurring. I have searched google, the docs, other SO posts, and found nothing.  </p>
  
  <p>Any ideas?</p>