SQLAlchemy supports creating [partial indexes in postgresql](http://docs.sqlalchemy.org/en/rel_0_9/dialects/postgresql.html#partial-indexes).

Is it possible to create a [partial **unique** index](http://www.postgresql.org/docs/current/interactive/indexes-partial.html#INDEXES-PARTIAL-EX3) through SQLAlchemy?

Imagine a table/model as so:

```py
class ScheduledPayment(Base):
     invoice_id = Column(Integer)
     is_canceled = Column(Boolean, default=False)

```

I'd like a unique index where there can be only one "active" ScheduledPayment for a given invoice.

I can create this manually in postgres:

```sql
CREATE UNIQUE INDEX only_one_active_invoice on scheduled_payment 
     (invoice_id, is_canceled) where not is_canceled;

```

I'm wondering how I can add that to my SQLAlchemy model using SQLAlchemy 0.9.