Creates an audit entry, which is much like a snapshot in that
        it references versions and provides some history of the changes made.

def _write_audit(self, user, message, changed_version):
        """
        Creates an audit entry, which is much like a snapshot in that
        it references versions and provides some history of the changes made.
        """
        audit = {'_id': bson.ObjectId(),
                 'user': user,
                 'message': message,
                 'symbol': changed_version.symbol
                 }
        orig_version = changed_version.orig_version.version
        new_version = changed_version.new_version.version
        audit['orig_v'] = orig_version
        audit['new_v'] = new_version
        # Update the versions to contain the audit
        mongo_retry(self._versions.update_many)({'symbol': changed_version.symbol,
                                                 'version': {'$in': [orig_version, new_version]}
                                                 },
                                                {'$addToSet': {'parent': audit['_id']}})
        # Create the audit entry
        mongo_retry(self._audit.insert_one)(audit)