On Thu, 2009-03-19 at 13:20 -0700, adrian wrote: > > The doc gives this example for querying an Entry model which has a > foreign key field to a Blog model. > > e = Entry.objects.get(id=2) > e.blog = some_blog > > Fine. Now if I do it in a loop like this: > > e = Entry.objects.all() > for entry in e: > entry.blog = some_blog > #do stuff here with Entry and Blog > > My question is, would this create a lot of SQL statements? When does > Django query the > foreign key field? Does it get all the blog entries at once with the > Entry.objects.all() query, > or does it do it one by one in the loop?
It does it one by one because Python and Django have a limitation in that they cannot predict the future. When processing entry.blog the first time around, it doesn't know that you will be wanting to access another entry.blog (for a different entry). The Entry.objects.all() query only retrieves Entry objects, not all the related objects. The example is a little contrived, since you wouldn't normally work like that if you had a *lot* of entries. On the other hand, for a small number of entries, even one SQL query per time around the loop simply isn't that horrible. > > How would I do that efficiently because I need to create a single dict > from the value of my model and all its foreign key models, so that I > can serialize it for Ajax? I'd start by reading the documentation. All of it, so that you know what is possible. That isn't intended to be rude, but we do put a lot of time into documenting this stuff. If you're up to the point of trying to optimise things at this level, it's reasonable to hope that you've read as much of the documentation as you can get your hands on it. In particular, have a look at select_related(), which is designed for exactly this purpose. In other cases, you can often reduce the number of queries by turning things around and, say, querying on blogs rather than entries (for some different style of query construction). Regards, Malcolm --~--~---------~--~----~------------~-------~--~----~ 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 django-users+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
