import random
import threading
import time

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    data = Column(String(10))
    bs = relationship("B", ) #bake_queries=False)  # turn off baked to fix


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey("a.id"))
    data = Column(String(10))

e = create_engine("mysql://scott:tiger@localhost/test", echo="debug")
Base.metadata.create_all(e)

s = Session(e)

s.add(A(bs=[B()]))
s.commit()

def worker():
    s = Session(e)
    a1 = s.query(A).first()
    while True:
        time.sleep(.001)
        bs = a1.bs
        assert len(bs) == 1

        s.expire(a1, ['bs'])

def chaos():
    while True:
        time.sleep(random.random())
        try:
            del A.bs.property._lazy_strategy._simple_lazy_clause
        except AttributeError:
            pass

threads = [threading.Thread(target=chaos)]  # or remove chaos to fix
threads.extend(threading.Thread(target=worker) for i in range(20))

for t in threads:
    t.start()

for t in threads:
    t.join()