👀 Views: 53 💬 Answers: 1 📅 Created: 2025-06-06

Flask SQLAlchemy DetachedInstanceError Python

I'm relatively new to this, so bear with me. I'm converting an old project and I'm converting an old project and I'm relatively new to this, so bear with me... I'm working with a `DetachedInstanceError` in my Flask application when trying to access a relationship property after a query. I have a `User` model with a one-to-many relationship to an `Order` model, and I'm using SQLAlchemy 1.4.23 with Flask 2.0.1. Here's a snippet of my models: ```python from flask_sqlalchemy import SQLAlchemy db = SQLAlchemy() class User(db.Model): id = db.Column(db.Integer, primary_key=True) name = db.Column(db.String(50), nullable=False) orders = db.relationship('Order', backref='user', lazy=True) class Order(db.Model): id = db.Column(db.Integer, primary_key=True) user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False) product_name = db.Column(db.String(100), nullable=False) ``` In my view, I'm trying to retrieve a user and access their orders like this: ```python @app.route('/user/<int:user_id>') def get_user(user_id): user = db.session.get(User, user_id) return {'name': user.name, 'orders': [order.product_name for order in user.orders]} ``` However, when I access the `/user/<user_id>` endpoint, I get the following behavior: ``` sqlalchemy.exc.DetachedInstanceError: Parent instance <User at 0x...> is not bound to a Session; maybe you need to refresh it or set up a new Session. ``` I attempted to fix it by using `db.session.refresh(user)` before accessing `user.orders`, but that didn't resolve the scenario. I'm not sure if it's related to my session management or the way I'm querying the database. Any insights on how to properly access related properties without causing this behavior? Also, I checked my Flask app's configuration and made sure that `SQLALCHEMY_TRACK_MODIFICATIONS` is set to `False` to avoid any overhead, but it doesn't seem to affect the scenario. Has anyone else encountered this? My development environment is CentOS. Thanks, I really appreciate it! This is for a mobile app running on CentOS. This is my first time working with Python latest. I'd really appreciate any guidance on this.