On Tue, 2008-04-15 at 07:54 -0700, robotika wrote: > I was initially excited at the idea of specifying a list related > fields with a select_related call, but have since discovered it > doesn't seem to work on reverse relations. I don't know why I was > expecting it to work (stupid optimism!), but it was a little > disappointing. What I was curious about, and I guess this question is > mostly for Malcolm, is how difficult would it be to allow > select_related to follow reverse relations if the fields/models were > specified? If it isn't too difficult, would this be something that > could end up in Django at some point?
Anything that is multi-valued is problematic because you have to (a) work out a way to represent the results (that bit's actually fairly easy and (b) make sure that people don't shoot themselves in the foot by trying to extract more than one multi-valued result at a time. Part (b) is a little tricky, because it's surprisingly easy to do by accident. The problem with multiple multi-valued results is that you cannot do it efficiently in a single SQL query without using UNION (and constructing the UNION to be efficient isn't completely trivial either in the general case). So it's not too hard to construct the naïve query for a something with multi-valued field #1 returning 1000 results and multi-valued field #2 returning 1000 results that ends up retrieving 1000 * 1000 = 1 000 000 rows, rather than only 2000 rows (you must query for the #1 and #2 fields separately). The same category of problem includes using many-to-many fields in select_related. It also includes doing the same thing in values() queries. Not impossible to solve, but important to solve it efficiently and not entirely trivial to do so. Therefore, something for The Future. Regards, Malcolm -- Remember that you are unique. Just like everyone else. http://www.pointy-stick.com/blog/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
