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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 _view_filter(self):
 """ Returns the mongodb catalog filter for isolated or federated views. This also searches across all underlying catalogs in federated catalog views. Real authz for controlling access to underlying catalogs will need to be managed in an adapter above the pay grade of this implementation. """

        if self._is_phantom_root_federated():
            return {}
        idstr_list = self._get_catalog_idstrs()
        return {'assigned' + self._catalog_name + 'Ids': {'$in': idstr_list}}