uniqueconstraint attachment logic fails for combination of Column + string names

from sqlalchemy import *
import random

m = MetaData()

columns = [Column('a', Integer), Column('b', Integer), Column('c', Integer)]
uq = UniqueConstraint(columns[0], 'b', 'c')

random.shuffle(columns)
t = Table('t', m, *(columns + [uq]))

based on the order of columns, the attach event associated with "a" will prematurely cause the constraint to initialize itself before the table is set up.

The original case arises due to a declarative mapping that combines column objects with string names, and the ordering changes due to changes in declared_attr:

class Foo(object):

    foo = Column(Unicode(64), nullable=False)

    @declared_attr
    def bar(cls):
        return Column("bar", Integer)


class ItemCategory(Base, Foo):
    __tablename__ = 'item_category'

    id = Column(Integer, primary_key=True)
    name = Column(Unicode(64), nullable=False)

    __table_args__ = (
        UniqueConstraint(name, "foo", "bar",
                         name='unique_category_and_parent'),)

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