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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 identifier(self):
 """These models have server-generated identifiers. If we don't already have it in memory, then assume that it has not yet been generated. """

        if self.primary_key not in self._data:
            return 'Unknown'
        return str(self._data[self.primary_key])