I have a Persistent derived class where I want to upgrade from using a PersistentList to a BTrees.IOBTree.IOBTree. _articleList is the old Persistent list _oidsToArticles is the new IOBTree. def __setstate__(self, state): articleList = state.get('_articleList') if articleList: oidsToArticles = self.makeBTree() for each in articleList: oidsToArticles[each.oid()] = each del state['_articleList'] state['_oidsToArticles'] = oidsToArticles Persistent.__setstate__(self, state) This seems to work fine, and I am adding more objects to _oidsToArticles. The problem is that when I commit the transaction, nothing is saved. I have tested the same code without __setstate__ and then it saves just fine. What hoops does one have to jump through to upgrade the state of a ZODB object?