Alembic 无法创建复合唯一索引

Question

我有以下模型：

class Table(Base):
    __tablename__ = 'table'
    __table_args__ = (
        Index('bar_baz_idx', 'bar', 'baz', unique=True,
              postgresql_where='foo IS NULL AND bar IS NOT NULL '
                               'AND baz IS NOT NULL'),
        {'schema': 'custom_schema'},
    )

    foo = Column(String(80), nullable=True)
    bar = Column(String(80), nullable=True)
    baz = Column(String(80), nullable=True)

当我自动生成 Alembic 迁移时，它会产生以下升级功能：

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('table',
    sa.Column('foo', sa.String(length=80), nullable=True),
    sa.Column('bar', sa.String(length=80), nullable=True),
    sa.Column('baz', sa.String(length=80), nullable=True),
    sa.Column('fxt', sa.String(), nullable=True),
    sa.Column('id', sa.Integer(), nullable=False),
    sa.PrimaryKeyConstraint('id'),
    schema='custom_schema'
    )
    op.create_index('bar_baz_idx', 'table', ['bar', 'baz'], unique=True, schema='custom_schema', postgresql_where='foo IS NULL AND bar IS NOT NULL AND baz IS NOT NULL')

但是，这失败了（在 Alembic 0.9.6 上），回溯如下：

  File "/Users/username/.virtualenvs/myproj/lib/python3.4/site-packages/SQLAlchemy-1.1.15-py3.4-macosx-10.6-intel.egg/sqlalchemy/dialects/postgresql/base.py", line 1727, in visit_create_index
    literal_binds=True)
  File "/Users/username/.virtualenvs/myproj/lib/python3.4/site-packages/SQLAlchemy-1.1.15-py3.4-macosx-10.6-intel.egg/sqlalchemy/sql/compiler.py", line 242, in process
    return obj._compiler_dispatch(self, **kwargs)
AttributeError: 'str' object has no attribute '_compiler_dispatch'

有什么明显的我在这里做错了吗？没有postgresql_where参数它似乎工作正常。