Thanks Mike. I must admit I don't understand why that code works, but it does. I guess that's the "Alchemy" in "SQLAlchemy" :-)
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. 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 properties and column_properties. On Jan 14, 4:23 pm, "Michael Bayer" <mike...@zzzcomputing.com> wrote: > bojanb wrote: > > Let's say I want to have a simple calculated property in my class, eg. > > amount which is just qty * price. > > > I can define it as a column_property in a mapper which makes it > > available in all database operations, eg. I can write session.query > > (myclass).filter_by(amount>1000) which will create the correct WHERE > > clause "qty*price>1000". > > > However, the attribute is None until the object is flushed to the > > database, ie. > > myclass.qty = 2 > > myclass.price = 500 > > print myclass.amount > > > will return None if flush was not issued. > > > If I use a Python property function to define it, it will be > > immediately available (and always up to date); however, I cannot query > > on a Python property. > > ultimately the value of this attribute is derived from other attributes > which are mapped. So there is a very simple and clever way to get both > in that case which you can see if you look at > examples/derived_attributes/attributes.py. > > > > > Is there a way to have best of both worlds? Or should I just define > > the calculated property twice, eg. Python property named 'amount' and > > a column_property named 'db_amount' and then work with the first but > > use the second for querying? > > -- > > You received this message because you are subscribed to the Google Groups > > "sqlalchemy" group. > > To post to this group, send email to sqlalch...@googlegroups.com. > > To unsubscribe from this group, send email to > > sqlalchemy+unsubscr...@googlegroups.com. > > For more options, visit this group at > >http://groups.google.com/group/sqlalchemy?hl=en.
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