For now I have an unoptimized solution working. I don't ever keep a
reference to the SQLA instance and in the reconstructor I reattach the
ephemeral state. Something like.
class User(db.Model):
__data_cache = dict()
@orm.reconstructor
def init_on_load(self):
data = User.__data_cache.get(self.id)
if data is None:
User.__data_cache[self.id] = 0
User.__data_cache[self.id] += 1
Rob
On Wednesday, February 12, 2020 at 9:43:37 PM UTC-6, Rob Rosenfeld wrote:
>
> Thanks for the thoughts. I'm going to digest, read docs, and experiment.
>
>
> On Wed, Feb 12, 2020 at 8:52 AM Mike Bayer <mik...@zzzcomputing.com
> <javascript:>> wrote:
>
>>
>>
>> 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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>
--
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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