On Aug 5, 2010, at 5:06 PM, Zippy P wrote:
> I have the following code:
>
> from objects import *
>
> class Device(Base):
> __tablename__="testdev"
> devtype = Column(Unicode(20), primary_key = True)
> __mapper_args__ = {'polymorphic_on': devtype}
> mac = Column(Unicode(128), primary_key = True)
> switch_mac = Column(Unicode(128), ForeignKey(mac))
> switch = relationship('Device', primaryjoin = 'Device.switch_mac ==
> Device.mac', backref=backref('connections'), remote_side = [mac])
> cablelength = Column(Float, nullable = True)
>
> class PC(Device):
> __mapper_args__ = {'polymorphic_identity': u'P'}
>
> class Switch(Device):
> __mapper_args__ = {'polymorphic_identity': u'S'}
>
> Switch.PC_cablelength_avg = column_property (
> select([func.avg(PC.cablelength)]).where(PC.switch_mac ==
> Switch.mac).as_scalar()
> )
> >>> s = Switch(mac='s1')
> >>> p = PC(mac='p1',switch_mac=s.mac, cablelength = 100)
> >>> Session.add(p) # adds a PC with a cable length of 100 to switch s
> >>> Session.add(s)
> >>> Session.commit()
> >>> s = Session.query(Switch).first()
> >>> s.PC_cablelength_avg
> >>>
>
> Expected value for s.PC_cablelength_avg is 100.
>
> The select statement (from first()) looks like this:
>
> SELECT testdev.devtype AS testdev_devtype, testdev.mac AS testdev_mac,
> testdev.switch_mac AS testdev_switch_mac, testdev.cablelength AS
> testdev_cablelength, (SELECT avg(testdev.cablelength) AS avg_1
> FROM testdev
> WHERE testdev.switch_mac = testdev.mac) AS anon_1
> FROM testdev
> WHERE testdev.devtype IN (%s)
> LIMIT 0, 1
> ('S',)
>
>
> The issue, I think, is that the "WHERE testdev.devtype IN 'S'" is causing the
> results to not include objects of type PC, which is what's used in the
> column_property.
Ultimately you're looking for a self-referential correlation here. In SQL,
that always implies using aliases. Your column_property needs to be against
aliased(PC) and not PC itself.
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