foo.py

            'current_translation': orm.relationship(
                ContentTranslation,
                primaryjoin=lambda: sql.and_(
                    ContentTranslation.content_id == Content.id,
                    ContentTranslation.language_id ==  get_current_request().locale_name
                ),
                lazy='joined',
                uselist=False,
                innerjoin=True,
                viewonly=True
            ),

            'translations': orm.relationship(
                ContentTranslation,
                back_populates='content',
                cascade='all, delete-orphan',
                #lazy='subquery',
                collection_class=attribute_mapped_collection('language_id')
            ),