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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 build_rdn(self):
 """ Build the Relative Distinguished Name for this entry. """

        bits = []
        for field in self._meta.fields:
            if field.db_column and field.primary_key:
                bits.append("%s=%s" % (field.db_column,
                                       getattr(self, field.name)))
        if not len(bits):
            raise Exception("Could not build Distinguished Name")
        return '+'.join(bits)