Thank you, Michael!
But could I achieve it somehow using hybrid properties and a case statement?
I tried something like:
@hybrid_property
def time(self):
return self._time or self.event.time
@time.expression
def time(cls):
return case([(cls._time != None, cls._time)], else_=cls.parent.time)
but that doesn't work:
AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator'
object has an attribute 'time'
Now again I would have to join the corresponding Parent class and filter
then, but I can't do that as part of the case statement
(http://stackoverflow.com/a/6994915/1175818), so I wondered of there could
be any other trick that sqlalchemy offers.
I have no problem in joining to the Parent class, I just don't know how I
could do it in the hybrid expression.
Regards,
Moritz
Am Samstag, 21. April 2012 16:15:50 UTC+2 schrieb Michael Bayer:
>
>
> On Apr 21, 2012, at 7:06 AM, Moritz Schlarb wrote:
>
>
> So basically, a Child object may set it's own value for time, but if it's
> not set, it uses the value from its parent.
> This works, but I would also like to be able to do something like:
>
> session.query(Child).filter(Child.parent.another_id==2)
>
> (Which would throw Neither 'InstrumentedAttribute' object nor 'Comparator'
> object has an attribute 'another_id')
>
>
> SQLAlchemy doesn't implicitly generate joins when you call attributes from
> relationship-bound attribute (here, Child.parent is a relationship-bound
> attribute and "another_id" is an attribute you're trying to call from it).
>
> instead, you need to think in SQL and tell SQLAlchemy that you'd like to
> join from Child to Parent:
>
> query(Child).join(Child.parent).filter(Parent.another_id==2)
>
> joining is covered in the tutorial at
> http://docs.sqlalchemy.org/en/latest/orm/tutorial.html#querying-with-joins.
>
> Also when you do a query from inside of a @property, you might want to use
> object_session() to get at the Session. A similar example to what you're
> doing is here:
>
>
> http://docs.sqlalchemy.org/en/latest/orm/relationships.html#building-query-enabled-properties
>
>
>
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