SQLAlchemy sessions error

问题

Background: Flask / Flask-SQLAlchemy / Flask-WTF, using declarative and scoped session

Simple POST operation:

@tas.route('/order_add', methods=['GET', 'POST'])  
def tas_order_add():  
    if request.method == 'POST':
        order_form = OrderForm()
        if order_form.validate_on_submit():
            order = Order()
            order_form.populate_obj(order)
            db_session.add(order)
            db_session.commit()

Now trying to run it I get an error:

InvalidRequestError: Object '' is already attached to session '1' (this is '2')

Changing add to merge solves the problem, but:

I don't know why do I have to merge an object while I just initiated it
If I do change add to merge and try to define one of the properties something in line
```
order = Order()
order_form.populate_obj(order)
order.order_status = OrderStatus.query.filter(OrderStatus.code=='PLACED').first()
db_session.merge(order)
db_session.commit()
```
I get the same error, just now on OrderStatus object

InvalidRequestError: Object '' is already attached to session '2' (this is '1')

Can someone point me where I'm doing something wrong because it's driving me nuts. I do have some experience with SQLAlchemy but it's the first time I see such a behaviour and I can't pinpoint the problem.

Searching all I found was a problem with double database session initialization but I don't belive it's this case.

EDIT

db_session is defined in separate file database.py with following content

from sqlalchemy.engine import create_engine
from sqlalchemy.ext.declarative.api import declarative_base
from sqlalchemy.orm.scoping import scoped_session
from sqlalchemy.orm.session import sessionmaker

engine = create_engine('sqlite:///fundmanager_devel.db', convert_unicode=True)
db_session = scoped_session(sessionmaker(autocommit=False,
                                         autoflush=False,
                                         bind=engine))
Base = declarative_base()
Base.query = db_session.query_property()

回答1:

seems a problem with session mix up, I used it as follows and it works:

from <project> import db
from <project.models import Category

category = QuerySelectField('category', query_factory=lambda: db.session.query(Category), get_pk=lambda a: a.id, get_label=lambda a: a.name)

回答2:

I had a similar problem with QuerySelectField:

forms.py :

class SurgeryForm(Form):
    study = QuerySelectField('Study',
                             query_factory=StudyGroup.query.all,
                             get_label='name')

models.py

class Animal(db.Model):
    study_id = db.Column(db.Integer, db.ForeignKey('study_group.id'))

class StudyGroup(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    animals = db.relationship('Animal', backref='group', lazy='dynamic')

views.py:

def do_surgery():
    form = SurgeryForm(request.form)

    if form.validate_on_submit():
        a = models.Animal()
        form.populate_obj(a)  # Gather the easy stuff automagically
        #a.group = form.data['study']  #FAILS!
        a.study_id = form.data['study'].id  #WORKS!

It seems that SQLAlchemy (or possibly Flask-SQLAlchemy or Flask-WTF) uses one session to gather the values in the QuerySelectField and another session to create the new Animal object.

If you try to attach the StudyGroup object to the Animal using the backref (Animal.group) you'll run into this problem (since the objects associated with different sessions). The workaround that I'm using is to set the foreign key (Animal.study_id) directly.

Clearly I'm a little late to the party with this answer, but I hope it helps someone!

回答3:

That's weird. Why are you creating the engine and declarative base explicitly if you're using flask-sqlalchemy? That's likely where your problem is. It looks like you have two engines and sessions running concurrently, that's why you got the error.

Instead of creating the engine explicitly, you should use just:

from flask.ext.sqlalchemy import SQLAlchemy

db = SQLAlchemy()

And inside your app factory:

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///fundmanager_devel.db'
db.init_app(app)

Then your base declarative model is db.Model, your session is in db.session, and you should let flask request context manage session creation.

Check the minimal application example in Flask-SQLAlchemy docs:

http://pythonhosted.org/Flask-SQLAlchemy/quickstart.html#a-minimal-application

This is how it's recommended by SQLAlchemy:

Most web frameworks include infrastructure to establish a single Session, associated with the request, which is correctly constructed and torn down corresponding torn down at the end of a request. Such infrastructure pieces include products such as Flask-SQLAlchemy, for usage in conjunction with the Flask web framework, and Zope-SQLAlchemy, for usage in conjunction with the Pyramid and Zope frameworks. SQLAlchemy strongly recommends that these products be used as available.

http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html

来源：https://stackoverflow.com/questions/19769841/sqlalchemy-sessions-error

标签

python

sqlalchemy

flask-sqlalchemy