Flask-SQLAlchemy의 Column에 subquery 사용

class Question(db.Model):
    ...
    answer_count = db.column_property(db.select([db.func.count(db.text('qid'))])
                                      .select_from(db.text('answer'))
                                      .where(db.text('qid == question.id'))
                                      .correlate_except(db.text('answer'))
                                      .label('answer_count'))