backrefs for polymorphic multi table inheritance doesn't work for relations between children

Given a hierarchy of: * Parent * Child1 * Child2

Where Child1 has a reference to Child2, creating Child1 with a reference to Child2 doesn't work (Child1 assumes it's primary key is that of Child2). When Child1 is created with NULL reference to Child2, correct primary key is created.

import sqlalchemy as sql

engine = sql.create_engine("sqlite://")
engine.echo=True
meta = sql.BoundMetaData(engine)

devices = sql.Table("devs", meta,
    sql.Column("dev_id", sql.types.Integer, primary_key=True),
    sql.Column("type", sql.types.String, nullable=False),
    sql.Column("name", sql.types.String, nullable=False)
)

r_groups = sql.Table("r_groups", meta,
    sql.Column("rg_id", sql.types.Integer, sql.ForeignKey("devs.dev_id"), primary_key=True)
)

routers = sql.Table("routers", meta,
    sql.Column("r_id", sql.types.Integer, sql.ForeignKey("devs.dev_id"), primary_key=True),
    sql.Column("group_id", sql.types.Integer, sql.ForeignKey("r_groups.rg_id")),
    sql.Column("rdata", sql.types.String)
)

class Device(object):
    pass

class RouterGroup(Device):
    pass

class Router(Device):
    pass

dev_join = sql.polymorphic_union(
    {
        "router": devices.join(routers),
        "r_group": devices.join(r_groups),
        "device": devices.select(devices.c.type == "device")
    }, None, "dev_pjoin"
)

device_mapper = sql.mapper(Device, devices,
    select_table=dev_join,
    polymorphic_on=dev_join.c.type,
    polymorphic_identity="device")
sql.mapper(RouterGroup, r_groups,
    inherits=device_mapper,
    polymorphic_identity="r_group")

sql.mapper(Router, routers,
    inherits=device_mapper,
    properties = {
        "group": sql.relation(RouterGroup, backref="routers") #doesn't work
#        "group": sql.relation(RouterGroup)
    },
    polymorphic_identity="router")


meta.drop_all()
meta.create_all()

session = sql.create_session(engine)

rg = RouterGroup()
rg.name = "rg1"
session.save(rg)

r1 = Router()
r1.name = "router1"
session.save(r1)

r2 = Router()
r2.name = "router2"
r2.group = rg
session.save(r2)

session.flush()

Where Child1 has a reference to Child2, creating Child1 with a reference to Child2 doesn't work (Child1 assumes it's primary key is that of Child2). When Child1 is created with NULL reference to Child2, correct primary key is created.

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