On Mon, Feb 23, 2009 at 4:10 PM, Sean Patrick Hogan <spho...@gmail.com>wrote:
> It isn't *an* extra SQL query that I'm worried about. It's that it's going > to be about 2,000 extra SQL queries in a loop. If it were just constructing > a page with one item and then its related items, that's not a problem to do > two queries, but when you're looping over 2,000 items and running an extra > query for all 2,000, it becomes a large performance issue. When you're > dealing with that many items, it does affect the viability of your > application. > > Rails can do this. However, the problem isn't a simple one to solve. I'll > probably look at their code sometime later this week. Going back to my > example, SQL is going to give you the list of tuples of scalars as Aaron > notes. This isn't a problem that SQL can solve since an SQL result set will > look like a spreadsheet and (for obvious reasons) you just can't have many > related objects shown in a spreadsheet. > > BUT, you can iterate over objects in python. So, let's say I had this > result set: > Article1, Comment1 > Article 1, Comment 4 > Article 1, Comment 8 > Article 2, Comment 3 > Article 2 Comment5 > > Ugly! But I could loop over that and aggregate them together as such: > > final_results = [] > for item in results: > if item.pk != last_item.pk: > final_results.append(item) > item.comment_set = [] > item.comment_set.append(#a comment object from those columns) > last_item = item > > Basically, each loop you look to see if it's the same article and so you > realise that you should only be appending the comment to the comments list. > That seems easy enough, but then what do you do when you have multiple > relations you're trying to get in the select_related? It gets a lot harder. > > Anyway, I'll look over the Rails source sometime later this week and see if > there is a simple enough pattern to port or maybe just hack a simple case > through for single calls for the reverse relationship which isn't so hard. > > One thing that would be nice, however, is if select_related() would raise > an exception when you tried to call it and passed in a name that wasn't a > foreign key (and therefore it couldn't get) so that you'd get an error > message rather than having that error message pass silently. > > Errors should never pass silently. > Unless explicitly silenced. > ~Zen of Python > > Well, I guess I have a project for this week! > Sean. > > > > On Mon, Feb 23, 2009 at 3:55 PM, Alex Gaynor <alex.gay...@gmail.com>wrote: > >> >> >> On Mon, Feb 23, 2009 at 3:49 PM, Torsten Bronger < >> bron...@physik.rwth-aachen.de> wrote: >> >>> >>> Hallöchen! >>> >>> Alex Gaynor writes: >>> >>> > On Mon, Feb 23, 2009 at 12:40 PM, sphogan <spho...@gmail.com> wrote: >>> > >>> >> [...] >>> >> >>> >> However, select_related() only works on the object the foreign >>> >> key is declared on >>> >> (http://docs.djangoproject.com/en/dev/ref/models/querysets/ #id4 >>> >> <http://docs.djangoproject.com/en/dev/ref/models/querysets/%0A#id4>; >>> >> specifically, "You can only refer to ForeignKey relations in the >>> >> list of fields passed to select_related.") Presumably this is >>> >> because it does an INNER JOIN rather than a LEFT OUTER JOIN and >>> >> would therefore miss articles with no comments. >>> >> >>> >> What I would like to be able to do is: >>> >> >>> >> Article.objects.select_related('comment_set').all() >>> >> >>> >> This seems like a somewhat simple case and there seems to be no >>> >> mention of it in the documentation anywhere. >>> > >>> > That's not possible, >>> >>> By principle, or just because Django can't do it (yet)? The related >>> feature request can be found at >>> <http://code.djangoproject.com/ticket/2238>. It it, I refer to >>> >>> http://rubynugs.blogspot.com/2008/07/couple-of-stupid-things-about-djano.html >>> where a Rails fan claims that Rails can do it. >>> >>> I don't know much about SQL so I really would like to know whether >>> this could be realised. >>> >>> Tschö, >>> Torsten. >>> >>> -- >>> Torsten Bronger, aquisgrana, europa vetus >>> Jabber ID: torsten.bron...@jabber.rwth-aachen.de >>> >>> >>> >>> >> I have no idea if it's possible, by my inclination is that it isn't since >> no one seems to have ponied up the SQL to make it happen :) . Further SQL >> generally returns a list of tuples of scalars(to use there python datatypes) >> SQL doesn't really ever return a collection, so my inclination is that it >> can't. Someone better versed in SQL can probably say for sure. Regardless >> it's still easy to get the data: >> >> p =Post.object.get(pk=2) >> p.comment_set.all() # all the comments for that post. >> >> Alex >> -- >> "I disapprove of what you say, but I will defend to the death your right >> to say it." --Voltaire >> "The people's good is the highest law."--Cicero >> >> >> > > > > Sean, If you figured out how exactly the SQL that makes the eager loading of reverse relations possible I'm sure there would be quite a few people interested in making it possible in Django(myself among them, though not because I have a use, I just like puzzles :) ). Alex -- "I disapprove of what you say, but I will defend to the death your right to say it." --Voltaire "The people's good is the highest law."--Cicero --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. 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