did you use Bookmark.objetcs.all().select_related() ? As this will create a join query... at least for your model you show in the message.... I have found that for very complex joins, you need to use raw query and not a Model.
On Thursday, April 25, 2013 10:44:28 AM UTC-4, Matthieu Bouron wrote: > > On Thursday, April 11, 2013 5:39:57 PM UTC+2, Tom Evans wrote: >> >> On Thu, Apr 11, 2013 at 3:42 PM, Matthieu Bouron >> <[email protected]> wrote: >> > Hello, >> > >> > Is there a way to handle many-to-many relationship with raw sql queries >> ? >> > I have the following model: >> > >> > from django.db import models >> > >> > class Tag(models.Model): >> > name = models.CharField(max_length=512, unique=True) >> > >> > class Bookmark(models.Model): >> > link = models.CharField(max_length=512) >> > title = models.CharField(max_length=512) >> > tags = models.ManyToManyField(Tag) >> > added_at = models.DateField() >> > >> > Using Bookmark.objetcs.all() will result in a subquery for each row of >> the >> > bookmark table which is not acceptable for performance reasons. >> > First question is there a way to fetch all the content with a single >> query ? >> > Second question is it possible to use raw sql queries. I tried the >> > following: >> > >> > bookmarks = Bookmark.objects.raw( >> > 'SELECT * FROM bookmarkmanager_bookmark b ' >> > 'LEFT JOIN bookmarkmanager_bookmark_tags bt ON ( >> b.id = >> > bt.bookmark_id) ' >> > 'LEFT JOIN bookmarkmanager_tag t ON (bt.tag_id = >> t.id)' >> > ) >> > >> > But when i tried to access the tags attribute on a bookmark i get the >> > following exception: >> > >> > ValueError: "<Bookmark: Bookmark object>" needs to have a value for >> field >> > "bookmark" before this many-to-many relationship can be used. >> > >> > Thanks in advance, >> > Matthieu >> > >> >> Are you tied to raw SQL queries? Django itself supports traversing M2M >> relationships in a sane manner if you tell it to do so: >> >> >> https://docs.djangoproject.com/en/1.5/ref/models/querysets/#prefetch-related >> > > Thanks. > Another problem remains ... performances are horrible even if i use > values(). > I have 1000 rows in base, executing from a script (db sqlite): > - 800ms to fetch all the objects (traversing m2m with prefetch_related). > - 200ms to fetch all the row from bookmark table without traversing the > m2m relationship). > > Are my results expected ? > Are there any plan on improving django orm performance ? > > Matthieu > -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-users?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
