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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 as_doc(self):
 """Get the SON document representation of this DBRef. Generally not needed by application developers """

        doc = SON([("$ref", self.collection),
                   ("$id", self.id)])
        if self.database is not None:
            doc["$db"] = self.database
        doc.update(self.__kwargs)
        return doc