class User(TableMixin, Base):
    username = Column(String, nullable=False, unique=False)
    password = Column(String, nullable=False, unique=False)

class BBTableMixin(BBTableBaseMixin):
    id = Column(Integer, primary_key=True)
    active = Column(Boolean, nullable=False, default=True)

CREATE UNIQUE INDEX unique_active_username on user (username, active)
where active = True;

Index('ix_active_username', "active", username,
      unique=True,
      postgresql_where=("active" is True)
      )