unincremented version id counter on superclass

from ~~#3673~~

from sqlalchemy import *
from sqlalchemy import types
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base,  declared_attr
from sqlalchemy import event

from datetime import datetime

Base = declarative_base()

class Foo(Base):
    __tablename__ = 'foo'
    id = Column(types.Integer, primary_key=True)
    type = Column(types.Unicode(64), nullable=False)
    bar = Column(types.DateTime(), nullable=False)
    other = Column(types.Integer)

    __mapper_args__ = {
        'polymorphic_on': u'type',
        'polymorphic_identity': u'Foo',
        'version_id_col': bar,
        'version_id_generator': False
    }

class FooDerived(Foo):
    __tablename__ = 'foo_derived'
    id = Column(types.Integer, ForeignKey(u'foo.id'), primary_key=True)
    other2 = Column(types.Integer)

    __mapper_args__ = {
        'polymorphic_identity': u'FooDerived'
    }


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

session = Session(e, autocommit=True, autoflush=False, expire_on_commit=False)

x = FooDerived()
x.bar = datetime(1970, 1, 1)
session.add(x)
session.flush()

x.other2 = 1
session.flush()

it tries to update the primary table:

#!

OperationalError: (sqlite3.OperationalError) near "WHERE": syntax error [SQL: u'UPDATE foo SET  WHERE foo.id = ? AND foo.bar = ?'] [parameters: (1, '1970-01-01 00:00:00.000000')]

docs:

#!


We can update our ``User`` object without incrementing the version counter
as well; the value of the counter will remain unchanged, and the UPDATE
statement will still check against the previous value.  This may be useful
for schemes where only certain classes of UPDATE are sensitive to concurrency
issues::

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