Need access to flask request context & db session after they exit

Question

I'm writing a flask app that streams content out to the user, and I'm trying to manipulate the db while this streaming is happening. Here's some example code (simplified):

def work_hard(obj):
    yield 'About to do a lot of work...'
    obj.status = do_a_lot_of_work_very_slowly()
    yield obj.status
    db.session.commit()
    obj.more = more_slow_stuff()
    yield obj.more
    db.session.commit()
    yield 'Hard work is done!'

@app.route('/log/<int:objid>/work_hard', methods=['POST'])
def perform_action(objid):
    obj = MyModel.query.get(objid)
    return Response(work_hard(obj), mimetype='text/html')

This code gives the error Instance <MyModel at 0x7f5555f046a0> is not bound to a Session; attribute refresh operation cannot proceed, but if I call db.session.commit() inside perform_action() instead of work_hard(), it works. Similarly, if I try to access flask's request instance, it works in perform_action() but not work_hard() (it complains that I'm trying to access the request outside of a request context).

I assume that these are both because work_hard() is executing after perform_action() has returned. Is is possible to somehow prolong the request context to include work_hard()? So far I've just been passing individual values from request to work_hard() and that worked to a point, but now I need to commit to the db and I'm not sure how to fix the db session here.

I can't just call db.session.commit() from perform_action(), I really do need to be able to make multiple updates in the db in real-time as the output is streaming to the HTTP client.

If this turns out not to be possible, my backup plan is to stream output from a subprocess, and then in the subprocess I'll connect to the db from there, but I'd prefer to do it all within the same process if possible.

Thanks!

univerio · Accepted Answer · 2016-02-19 20:18:45Z

2

You can use stream_with_context() to keep the context (and hence the session) around while the generator runs:

return Response(stream_with_context(work_hard(obj)), ...)

answered Feb 19, 2016 at 20:18

univerio

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robru Over a year ago

This is it, thank you! Quick note for googlers in the future: My first attempt got me the dreaded

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such savepoint: sa_savepoint_1 [SQL: 'RELEASE SAVEPOINT sa_savepoint_1']

and googling this suggested some pretty ugly workarounds, but those workarounds are for older versions of python. in python 3.5 at least, all i had to do was stop calling db.session.begin_nested() which was a failed experiment I'd forgotten to delete anyway. It works!

Cani · Accepted Answer · 2016-02-19 21:58:55Z

1

Access the session with application context: with app.app_context

def work_hard(obj):
    yield 'About to do a lot of work...'
    obj.status = "blub"
    yield obj.status
    with app.app_context():
        db.session.commit()
    obj.more = "fish"
    yield obj.more
    with app.app_context():
        db.session.commit()
    yield 'Hard work is done!'

@app.route('/log/<int:objid>/work_hard', methods=['POST'])
def perform_action(objid):
    def perform_action(objid):
    obj = MyModel.query.get(objid)
    return Response(work_hard(obj), mimetype='text/html')

answered Feb 19, 2016 at 21:58

Cani

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