INSERTs and UPDATEs break when polymorphic discriminator is part of the primary key

It appears that INSERTs and UPDATEs do not work under the following conditions: 1. Table with composite primary key. 2. A hierarchy of classes polymorphically mapped to the table (single inheritance) 3. The polymorphic discriminator column is part of the primary key.

Also, while the 'insert' problem can be worked around (see bellow), fixing the 'update' requires patching SQLAlchemy's code.

Here's a test case:

from sqlalchemy import *
from sqlalchemy.orm import *

engine = create_engine('sqlite://')
metadata = MetaData()

test_table = Table("test", metadata,
    Column("id1", Integer, primary_key=True),
    Column("id2", UnicodeText, primary_key=True),
    Column("data", UnicodeText)
)

class TestBase(object):
    pass

class TestDeriv(TestBase):
    pass

mapper(TestBase, test_table,
    polymorphic_on=test_table.c.id2,
    polymorphic_identity=u"base")

mapper(TestDeriv,
    inherits=TestBase,
    polymorphic_on=test_table.c.id2,
    polymorphic_identity=u"deriv")

metadata.create_all(bind=engine)
session = sessionmaker(bind=engine, autoflush=False)()

t1 = TestDeriv()
t1.id1 = 0
# id2 is implied by instance's type, but we still
# have to assign it explicitly in order the insert to work
t1.id2 = u"deriv"

session.add(t1)
session.commit()
session.clear()
del t1

t1 = session.query(TestDeriv).get((0, u"deriv"))
t1.data = u"foo bar"

# The following line raises ConcurrentModificationError
# Logged SQL shows that id2 is bound to None
session.commit()

The fix (see the attached file, works against rev. 5098) is next to trivial (it only modifies two lines), but I have no way to be sure that it won't break something else.

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