[Sqlalchemy-tickets] Issue #4169: Inconsistent Execution order for "mapper_configured" (zzzeek/sqlalchemy)

[Lukas S. <iss...@bi...>]

New issue 4169: Inconsistent Execution order for "mapper_configured"
https://bitbucket.org/zzzeek/sqlalchemy/issues/4169/inconsistent-execution-order-for

Lukas Siemon:

We have been getting random failures. I narrowed this down to non deterministic execution order. I've included a (somewhat) minimal test case below.

The way the test case is currently set up it sometimes fails and other times it doesn't (might depend on compilation order?). If I comment the first `sleep(1)` the test always fails. If I comment the second `sleep(1)` the test never fails.

We would expect the execution order to be consistent.

```python
import unittest
from time import sleep
from sqlalchemy import Column, Integer, event, literal, func, select
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import mapper, column_property
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = ("sqlite://")
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

Base = declarative_base()


class Venue(db.Model):
    __tablename__ = 'venue'
    id = Column(Integer, primary_key=True, nullable=False)


class Offer(db.Model):
    __tablename__ = 'offer'
    id = Column(Integer, primary_key=True, nullable=False)


@event.listens_for(mapper, "mapper_configured")
def mapper_listener(mapper_, cls):
    if issubclass(cls, Venue):
        sleep(1)
        cls.info = column_property(
            select([
                func.SUM(Offer.info)
            ]).where(
                Offer.id <= Venue.id
            ).as_scalar()
        )

@event.listens_for(mapper, "mapper_configured")
def mapper_listener(mapper_, cls):
    if issubclass(cls, Offer):
        sleep(1)
        cls.info = column_property(cls.id + literal("1"))

db.drop_all()
db.create_all()


class TestExecutionOrder(unittest.TestCase):

    def test_execution_order(self):

        db.session.add(Venue())
        db.session.add(Offer())
        db.session.commit()

        venue = Venue.query.one()
        print venue.info
```

Stacktrace when test fails:
```sh
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File ".../test.py", line 53, in test_execution_order
    db.session.add(Venue())
  File "<string>", line 2, in __init__
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 347, in _new_state_if_none
    state = self._state_constructor(instance, self)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 767, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 177, in _state_constructor
    self.dispatch.first_init(self, self.class_)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/event/attr.py", line 256, in __call__
    fn(*args, **kw)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 3123, in _event_on_first_init
    configure_mappers()
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 3016, in configure_mappers
    mapper, mapper.class_)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/event/attr.py", line 218, in __call__
    fn(*args, **kw)
  File ".../env/local/lib/python2.7/site-packages/sqlalchemy/orm/events.py", line 621, in wrap
    fn(*arg, **kw)
  File ".../test.py", line 33, in mapper_listener
    func.SUM(Offer.info)
AttributeError: type object 'Offer' has no attribute 'info'
```

Running latest PostgreSQL 9.6