> > However, I have issues with the difference in NULL value semantics > > between Python and SQL. Ie. if a calculated column is defined via a > > column_property as price*amount, then the result will be NULL if any > > of the values is NULL. However, in Python, None*something throws a > > TypeError, so the hybrid_property getter function needs to be filled > > with lots of IFs. > > When called as class properties, the descriptors always generate > SQL expressions as above. > > When called as instance properties, it just calls your function and > you can > do what you like with None values, e.g.: > > @hybrid_property > def length(self): > return self.thing is not None and self.thing or None
That's what I did, I just wasn't sure it's the right way to do it. > > Also, this solution can't be used for date calculations, as timedelta > > objects are needed. So I guess I will stick with a mix of Python > > The example works in postgresql with timestamps and intervals/ > timedeltas. ;-) > > a. Then I'm doing something terribly wrong. If I define: due_date = hybrid_property(lambda self: self.invoce_date + self.payment_days) When querying with session.query(Invoice).filter (Invoice.due_date<date.today()).all(), I get the following error: TypeError: unsupported type for timedelta days component: InstrumentedAttribute This is with PostgreSQL and SQLAlchemy 0.5.8.
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