Read the targets value

def read(self):
        """
        Read the targets value
        """
        cursor = self.get_collection().find(
            {
                '_id': {'$in': self._document_ids},
                self._field: {'$exists': True}
            },
            {self._field: True}
        )

        return {doc['_id']: doc[self._field] for doc in cursor}