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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 ensure_fields(self, fields, force_refetch=False):
 """ Makes sure we fetched the fields, and populate them if not. """


        # We fetched with fields=None, we should have fetched them all
        if self._fetched_fields is None or self._initialized_with_doc:
            return

        if force_refetch:
            missing_fields = fields
        else:
            missing_fields = [f for f in fields if f not in self._fetched_fields]

        if len(missing_fields) == 0:
            return

        if "_id" not in self:
            raise Exception("Can't ensure_fields because _id is missing")

        self.refetch_fields(missing_fields)