Avoid DetachedInstanceError in Flask-SQLAlchemy

gistfile1.py

# The following produces DetachedInstanceError

app = Flask(__name__)
db = SQLAlchemy(app)

class Foo(db.Model):
    id = db.Column(db.Integer, primary_key=True)
  
foo = Foo()
db.session.add(foo)
db.session.commit()
foo.id

>>> DetachedInstanceError: Instance <Foo at 0x10741ac10> is not bound to a Session; attribute refresh operation cannot proceed

# The following removes DetachedInstanceError

app = Flask(__name__)
db = SQLAlchemy(app, session_options{
    'expire_on_commit': False
})

class Foo(db.Model):
    id = db.Column(db.Integer, primary_key=True)
  
foo = Foo()
db.session.add(foo)
db.session.commit()
foo.id

>>> 1

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

# The following produces DetachedInstanceError

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
db = SQLAlchemy(app)
# db = SQLAlchemy(app, session_options={
#     'expire_on_commit': False
# })

class Foo(db.Model):
    id = db.Column(db.Integer, primary_key=True)

db.create_all()
foo = Foo()
db.session.add(foo)
db.session.commit()
print(foo.id)

I have ran the above code on my computer, but it didn't raise a DetachedInstanceError，the flask version on my computer is:

Flask==0.12
Flask-SQLAlchemy==2.2
SQLAlchemy==1.1.6

I have troubleshooting this issue half day, this is realy help me.
thank you very much！

Thank you very much help. This helped to solve my problem.

But you don't close the session, the commit operation just expire the orm object, why does this lead to produces DetachedInstanceError ?
flask-sqlalchemy auto close when commit ?

Gosh this is a blast from the past @kalelsun 😄 I no longer work with Python or Flash / SQL Alchemy so I can't really answer your question, sorry, of course this very old code at this point so I have no idea if it is still relevant.