on update cascade changes PK, mapper can't find the row

technically, this can't work in any case due to version_id_col, which will be inconsistent. but when removed, the mapper still attempts to update the rows whose PKs have changed - it any case, this needs to be supported so that other row values can be reached on update.

basically this is the one case where _save_obj() has to look at history.added instead of history.deleted when looking for a mutated primary key value. Somehow the dependency rules need to put a clue in there.

import sqlalchemy as sql
from sqlalchemy import orm
from sqlalchemy.ext.declarative import declarative_base

__metaclass__ = type

engine = sql.create_engine("postgresql://scott:tiger@localhost/test", echo=True)
metadata = sql.MetaData(bind=engine)
db = orm.create_session(bind=engine, autocommit=False)
T = declarative_base(metadata=metadata)

class P(T):
   __tablename__ = 'p'
   id = sql.Column(sql.String, primary_key=True)
   version = sql.Column(sql.Integer, nullable=False, default=1)
   cc = orm.relation('C', backref='parent', passive_updates=True)
   __mapper_args__ = {'version_id_col': version}

class C(T):
   __tablename__ = 'c'
   i = sql.Column(sql.String, primary_key=True)
   p = sql.Column(sql.String,
                  sql.ForeignKey('p.id', 
                                onupdate='cascade',
                                ondelete='cascade'),
                                primary_key=False)
   version = sql.Column(sql.Integer, nullable=False, default=1)

   __mapper_args__ = {'version_id_col': version}

metadata.create_all()
P.__table__.delete().execute()

with db.transaction:
   p = P(id='P1', cc=[C(i='C.2')](C(i='C.1'),))
   db.add(p)

#db.expunge_all()
#p = db.query(P).first()

#with db.transaction:
#   p.id = 'P2'
   # ok, no ConcModError

#db.expunge_all()
#p = db.query(P).first()

with db.transaction:
   p.id = 'P3'
   p.cc
   # issues spurious updates, throws ConcModError

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