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
>
> >
>
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