<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def url(self):
 """ Constructs and returns the Query URL. :returns: Query URL """

        if self._partition_key:
            base_url = self._database.database_partition_url(
                self._partition_key)
        else:
            base_url = self._database.database_url

        return base_url + '/_find'