How to make a sqlalchemy database session per request in Flask? - flask

Question

I need to ensure transactionality with flask requests. Is there a way to store a database session per request and use that one?

Answer 1

The easiest way is to create a simple extension for session initialization and commit changes after successful request. Here is an example:
from datetime import datetime
from flask import Flask, current_app, _app_ctx_stack
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base, DeferredReflection
from sqlalchemy import Column, String, Integer
# Base model without connection(see: DBExtension.connect()) and a few demo models
Base = declarative_base(cls=DeferredReflection)
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True, autoincrement=True)
name = Column(String())
class Log(Base):
__tablename__ = 'log'
id = Column(Integer, primary_key=True, autoincrement=True)
text = Column(String())
# extension for getting db session
class DBExtension:
def __init__(self, app: Flask = None):
self.app = app
if app is not None:
self.init_app(app)
def init_app(self, app):
app.teardown_appcontext(self.teardown)
def connect(self):
# init engine and set into metadata
engine = create_engine(current_app.config['DB_CONNECTION'], echo=True)
Session = sessionmaker(bind=engine)
session = Session()
# create_all - just for demo(init structure...)
Base.metadata.create_all(engine)
Base.metadata.bind = engine
Base.prepare(engine)
return session
def teardown(self, exception):
ctx = _app_ctx_stack.top
if hasattr(ctx, 'session'):
ctx.session.close()
#property
def session(self):
# connect or get active connection
ctx = _app_ctx_stack.top
if ctx is not None:
if not hasattr(ctx, 'session'):
ctx.session = self.connect()
return ctx.session
app = Flask(__name__)
app.config.update({'DB_CONNECTION': 'sqlite:///test.db'}) # sqlite for demo
db = DBExtension(app)
#app.teardown_request
def teardown_request_func(error=None):
# request processing completed(before requests functions, endpoints and other logic)
if not error:
# one more record and commit all changes in one transaction
db.session.add(Log(text='{dt}. teardown_request'.format(dt=datetime.utcnow())))
try:
db.session.commit()
except Exception as e:
print(e)
# do something
#app.before_request
def before_request_func():
# add a new record before request
db.session.add(Log(text='{dt}. before request'.format(dt=datetime.utcnow())))
#app.route('/user/<name>')
def user_route(name: str):
# do something with db session
db.session.add(Log(text='saving user {user}'.format(user=name)))
user = User(name=name)
db.session.add(user)
# session.add(newModel) or session.delete(oneMoreModel) etc.
return 'done' # return without commit. see: teardown_request_func
if __name__ == '__main__':
app.run(host='localhost', port=5000, debug=True)
Run server, open http://localhost:5000/user/DavidBoshton and see logs(BEGIN-...-COMMIT).