flask-sqlalchemy timeouterror by queuepool overflow using backref option

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i'm beginner with flask-sqlalchemy. i encountered a problem maybe caused by using backref.

the api(view api. it renders template) is like this.

@board.route('/')
def index():
page = request.args.get('page', 1, type=int)
q = Article.query.order_by(Article.create_date.desc())
article_list_pagination = q.paginate(page, per_page=10)
return render_template('board.html', article_list=article_list_pagination)

and the model is like this

class Answer(db.Model):
__tablename__ = 'answer'
id = db.Column(db.Integer, primary_key=True)
article_id = db.Column(db.Integer, db.ForeignKey('article.id', ondelete='CASCADE'))
article = db.relationship('Article', backref=db.backref('answer_set'))
user_id = db.Column(db.String(255), nullable=False)
name = db.Column(db.String(255), nullable=False)
content = db.Column(db.Text(), nullable=False)
create_date = db.Column(db.DateTime(), nullable=False)
update_date = db.Column(db.DateTime(), nullable=True)

when i request the api every 20 times, i get this error

sqlalchemy.exc.TimeoutError: QueuePool limit of size 10 overflow 10 reached, connection timed out, timeout 30.00 (Background on this error at: https://sqlalche.me/e/14/3o7r)

here is what i tried to solve the error.

  1. give lazy options to db.relationship. but doesn't work
  2. db.session.close(). but it makes another error

sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <Article at 0x1772756baf0> is not bound to a Session; lazy load operation of attribute 'answer_set' cannot proceed

  1. debug=False. it doesn't work, too.

how can i get over this problem?

pythonflasksqlalchemyflask-sqlalchemy