On Tue, Feb 11, 2020, at 11:41 PM, Rob Rosenfeld wrote:
> Hi All,
>
> I am using Flask-SQLAlchemy on a legacy database. My SQLA classes / tables
> are setup using declarative base and autoload. In addition to using the class
> to access persisted data, I also use plain old Python to enrich them with
> ephemeral data.
>
> Every time I load a User object, I'd like the instance for that particular
> object / row to retain its plain old python instance data.
>
> My understanding is that Flask-SQLAlchemy will by default use a separate
> scoped_session for each request. And that means at the end of the request
> session and user will go out of scope. So in the next request to /users/Bob
> will instantiate a new User object whose will be reinitialized . . . each
> time I make this request I will be told it was "asked 1 time".
>
> Alternately, if I call /users_alt/Bob, then second time I call a
> DetachedInstanceError will be raised when I attempt to read user.owners.
>
> I suspect there's a pattern used to accomplish what I want to do, but I have
> not tried the right Google search. Any recommendations on how to approach
> this or searches to try / doc sections to read? The state I'm storing in
> times_name_read_since_launch is actually complex enough that I don't want to
> persist it and have to keep that dependent state synchronized.
>
> Alternately is there a way for me to reattach a SQLA object to the current
> requests' session? Then, ideally, I could get updates from the database to
> User or Owner objects, but retain my ephemeral state in the plain old python.
you can get a plain reattach without it emitting any SQL using session.add():
session.add(detached_object)
however, if you're not sure if detached_object's primary key might have been
loaded already, you would want to use session.merge() instead. if your object
has no changes on it you can add load=False which will avoid a SQL round trip.
>
> Thanks for the help,
> Rob
>
> from flask import Flask
> from flask_sqlalchemy import SQLAlchemy
> from sqlalchemy import MetaData
> from sqlalchemy import orm
>
> app = Flask(__name__)
> app.config['SQLALCHEMY_DATABASE_URI'] = connection_string
> db = SQLAlchemy(app)
>
>
> class Owner(db.Model):
> __table__ = db.Table('Owners', MetaData(), autoload=True,
> autoload_with=db.engine)
>
>
> class User(db.Model):
> __table__ = db.Table('users', MetaData(), autoload=True,
> autoload_with=db.engine)
> owners = db.relationship('Owner',
> primaryjoin="foreign(User.userId)==remote(Owner.userId)", uselist=True,
> backref=db.backref('user', uselist=False))
>
> users_seen = dict()
>
> @orm.reconstructor
> def init_on_load(self):
> self.times_name_read_since_launch = 0
>
> @classmethod
> def lookup_by_name(cls, name):
> user_ = User.users_seen.get(name)
> if user_ is None:
> user_ = User.query.filter(User.UserName == name).one()
> return user_
>
> @classmethod
> def store_by_name(cls, user_):
> if user_.UserName not in User.users_seen:
> User.users_seen[user_.UserName] = user_
>
> @property
> def name(self):
> self.times_name_read_since_launch += 1
> return self.UserName
>
>
> @app.route("/users/<string:name>")
> def user(name):
> user_ = User.query.filter(User.UserName == name).one()
> return "{} with {} - asked {} times".format(user_.name, user_.owners,
> user_.times_name_read_since_launch)
>
>
> @app.route("/users_alt/<string:name>")
> def user_alt(name):
> user_ = User.lookup_by_name(name)
> User.store_by_name(user_)
> owners = None
> if user_.times_name_read_since_launch > 0:
> # don't lazy load addresses the first request, simulates more complex actual
> behavior desired
> owners = user_.owners
> return "{} with {} - asked {} times".format(user_.name, owners,
> user_.times_name_read_since_launch)
>
>
> db.create_all()
>
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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