Michael Bayer wrote: > Kent wrote: >> There might be a communication problem, which an example could help >> clarify. >> >> I'm constrained by a legacy database that I have no control over >> changing. When an order is changed, I need to calculate the change in >> volume (or points) so I can update a table that records this >> information. >> >> Here is an example of a method you might want to run on an the >> original, unchanged object (Order, in this case): >> ================================================================= >> def calc_points(self): >> return sum(l.product.points * l.qtyordered for l in >> self.orderdetails if l.product.points is not None) >> ================================================================= >> >> Notice that this calculation relies on several relations: >> orderdetails, and orderdetails[].product >> >> From an MVC view point, my argument is that certainly this business >> logic *should* reside in the Order class. That is to say, "an Order >> should know how to calculate its own volume". >> >> Unfortunately, this is not a matter of two or three *fields* that I >> can easily extract from attributes.get_history(). This computation, >> again, relies on several relations. >> >> Any further insight or advice? >
also in case this is not apparent, this is exactly the kind of thing I do with SessionExtensions. For example if you look at examples/versioning/history_meta.py in the distribution you'd see exactly this technique using SessionExtension.before_flush() to create new "version" entries for objects being modified or deleted. Here's another example from a Pycon tutorial in 2009: http://bitbucket.org/zzzeek/pycon2009/src/tip/chap5/sessionextension.py - this one modifies a "count" attribute on a parent table in response to changes in a child collection, using after_flush(). we've hopefully implemented enough hooks into mapper and session to allow any possible change-based operation. there should be no need add esoteric hacks into methods like merge(). -- 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.
