Re-create grams for database.

        In normal situations, you never need to call this method.
        But after migrate DB, this method is useful.

        :param session: DB session
        :type session: :class:`sqlalchemt.orm.Session`

def recreate_grams(self):
        """Re-create grams for database.

        In normal situations, you never need to call this method.
        But after migrate DB, this method is useful.

        :param session: DB session
        :type session: :class:`sqlalchemt.orm.Session`

        """

        session = self.Session()

        for document in session.query(Document).all():
            logger.info(document.text)
            grams = self._get_grams(session, document.text, make=True)
            document.grams = list(grams)

        broken_links = session.query(Gram) \
            .filter(~Gram.documents.any()).all()
        for gram in broken_links:
            session.delete(gram)

        session.commit()