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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def doc_id(self):
 """Returns the couchbase document's id, object property. :returns: The document id (that is created from :attr:'doc_type' and :attr:'__key_field__' value, or auto-hashed document id at first saving). :rtype: unicode """

        if self.id:
            return '%s_%s' % (self.doc_type, self.id.lower())
        return self._hashed_key