SQLALchemy "after_insert" doesn't update target object fields - postgresql

Question

I have a model (see code below) on which I want to execute a function after an object is inserted that will update one of the object's fields. I'm using the after_insert Mapper Event to do this.
I've confirmed that the after_insert properly calls the event_extract_audio_text() handler, and the target is getting updated with the correct audio_text value. However, once the event handler finishes executing, the text value is not set for the object in the database.
Code
# Event handler
def event_extract_audio_text(mapper, connect, target):
# Extract text from audio file
audio_text = compute_text_from_audio_file(target.filename)
# Update the 'text' field with extracted text
target.audio_text = audio_text
# Model
class SoundsRaw(db.Model):
__tablename__ = 'soundsraw'
id = db.Column(db.BigInteger(), primary_key=True, autoincrement=True)
filename = db.Column(db.String(255))
audio_text = db.Column(db.Text())
# Event listener
event.listen(SoundsRaw, 'after_insert', event_extract_audio_text)
I've also tried calling db.session.commit() to try to update the object with the text value, but then I get the following stack trace:
File "/Users/alexmarse/.virtualenvs/techmuseum/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 219, in _assert_active
raise sa_exc.ResourceClosedError(closed_msg)
ResourceClosedError: This transaction is closed
Any ideas?
Software versions
SQLAlchemy 0.9.4
Flask 0.10.1
Flask-SQLAlchemy 1.0

Answer 1

The thing with 'after_insert' kind of handlers is to use the connection directly. Here's how I did it:
class Link(db.Model):
"News link data."
__tablename__ = 'news_links'
id = db.Column(db.BigInteger, primary_key=True)
slug = db.Column(db.String, unique=True) #, nullable=False
url = db.Column(db.String, nullable=False, unique=True)
title = db.Column(db.String)
image_url = db.Column(db.String)
description = db.Column(db.String)
#db.event.listens_for(Link, "after_insert")
def after_insert(mapper, connection, target):
link_table = Link.__table__
if target.slug is None:
connection.execute(
link_table.update().
where(link_table.c.id==target.id).
values(slug=slugify(target.id))
)

Answer 2

I ended up solving this by ditching the Mapper Event approach and using Flask's Signalling Support instead.
Basically, you can register "signals" on your model, which are essentially callback functions that are called whenever a specific kind of event happens. In my case, the event is an "update" on my model.
To configure the signals, I added this method to my app.py file:
def on_models_committed(sender, changes):
"""Handler for model change signals"""
for model, change in changes:
if change == 'insert' and hasattr(model, '__commit_insert__'):
model.__commit_insert__()
if change == 'update' and hasattr(model, '__commit_update__'):
model.__commit_update__()
if change == 'delete' and hasattr(model, '__commit_delete__'):
model.__commit_delete__()
Then, on my model, I added this function to handle the update event:
# Event methods
def __commit_update__(self):
# create a new db session, which avoids the ResourceClosedError
session = create_db_session()
from techmuseum.modules.sensors.models import SoundsRaw
# Get the SoundsRaw record by uuid (self contains the object being updated,
# but we can't just update/commit self -- we'd get a ResourceClosedError)
sound = session.query(SoundsRaw).filter_by(uuid=self.uuid).first()
# Extract text from audio file
audio_text = compute_text_from_audio_file(sound)
# Update the 'text' field of the sound
sound.text = audio_text
# Commit the update to the sound
session.add(sound)
session.commit()
def create_db_session():
# create a new Session
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
psql_url = app.config['SQLALCHEMY_DATABASE_URI']
some_engine = create_engine(psql_url)
# create a configured "Session" class
session = sessionmaker(bind=some_engine)
return session