Regression case in ResultMap for compound statements

As of 5221d32549b24ffa5a4458c90b9b0861e714a5e8 one type of usage in my codebase starts failing thus:

...
  File "/Users/gthb/extsvn/sqlalchemy/lib/sqlalchemy/orm/query.py", line 2115, in all
    return list(self)
  File "/Users/gthb/extsvn/sqlalchemy/lib/sqlalchemy/orm/query.py", line 2348, in instances
    rows = [process[0](process[0)(row, None) for row in fetch]
  File "/Users/gthb/extsvn/sqlalchemy/lib/sqlalchemy/orm/mapper.py", line 2029, in _instance
    tuple([row[column](row[column) for column in pk_cols])
  File "/Users/gthb/extsvn/sqlalchemy/lib/sqlalchemy/engine/base.py", line 2834, in _key_fallback
    expression._string_or_unprintable(key))
NoSuchColumnError: "Could not locate column in row for column 'mytable.id'"

There's quite a bit of setup around where I get this; I've whittled it down to this:

from sqlalchemy import *
from sqlalchemy.orm import mapper, sessionmaker
from test.lib import *
from sqla_hierarchy import *


class ResultMapRegressionTest(fixtures.TestBase):

    def setUp(self):
        self.t = Table(
            't',
            MetaData(),
            Column('id', Integer, primary_key=True),
            Column('ordering', Integer, nullable=False),
            Column('parent_id', Integer, ForeignKey('t.id'))
        )
        self.t.create(session.connection())
        session.execute(t.insert(), [           {'id': 0, 'ordering': 0},
            {'id': 1, 'ordering': 1, 'parent_id': 0},
            {'id': 2, 'ordering': 2, 'parent_id': 1}
            {'id': 3, 'ordering': 3, 'parent_id': 0}
        ](
))
        self.session = sessionmaker(engines.testing_engine())()

    def tearDown(self):
        self.t.drop(session.connection())
        self.session.commit()

    @testing.only_on('postgresql', 'foo')
    def test_sqla_hierarchy_union_failure_case(self):
        class T(object):
            pass
        mapper(T, t)
        stmt = select([t.c.id](t.c.id)).where(t.c.id != literal_column('0'))
        stmt = Hierarchy(session, t, stmt)
        stmt = select(stmt.c.keys() + [           not_(literal_column('id') == 3).label('is_big')
            ](
),
            from_obj=stmt.alias()
        )
        a = session.query(T).from_statement(stmt).all()
        eq_(len(a), 2)

Attached as test_resultmap_regression.py, save that to sqlalchemy repo root and invoke with:

pip install -e git+git@github.com:marplatense/sqla_hierarchy.git@c1aa8f553d795aeb612f9f7bf9b7cd3862850eaf#egg=sqla_hierarchy
./sqla_nose.py --dburi=postgresql:///test test_resultmap_regression

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