[Sqlalchemy-tickets] Issue #4171: list assignment does not seem to be affecting remove from previous object correctly (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4171: list assignment does not seem to be affecting remove from previous object correctly
https://bitbucket.org/zzzeek/sqlalchemy/issues/4171/list-assignment-does-not-seem-to-be

Michael Bayer:

major regression, needs immediate release


```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(Integer, ForeignKey('a.id'))
    a = relationship('A', backref=backref('bs'))

e = create_engine("sqlite://", echo='debug')
Base.metadata.create_all(e)

session = Session(e)

for i in xrange(100):
    a1 = A()
    a2 = A()

    b = B(a=a1)  # Set B.a = a1
    a2.bs = [b]  # ... but then put b onto a2's list

    session.add_all([a1, a2])
    session.commit()

    print len(a1.bs)


```

a1.bs is not consistently zero, depending on which of a1 or a2 is processed first.  list assignment not updating state correctly.    http://docs.sqlalchemy.org/en/latest/changelog/migration_12.html#a-validates-method-receives-all-values-on-bulk-collection-set-before-comparison is likely cause