Django Generic Relations and ORM Queries
Say I have the following models:
class Image(models.Model):
image = models.ImageField(max_length=200, upload_to=file_home)
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = generic.GenericForeignKey()
class Article(models.Model):
text = models.TextField()
images = generic.GenericRelation(Image)
class BlogPost(models.Model):
text = models.TextField()
images = generic.GenericRelation(Image)
What's the most processor- and memory-efficient way to find all Articles that have at least one Image attached to them?
I've done this:
Article.objects.filter(pk__in=Image.objects.filter(content_type=ContentType.objects.get_for_model(Article)).values_list('object_id', flat=True))
Which works, but besides being ugly it takes forever.
I suspect there's a better solution using raw SQL, but that's beyond me. For what it's worth, the SQL generated by the above is as following:
SELECT `issues_article`.`id`, `issues_article`.`text` FROM `issues_article` WHERE `issues_article`.`id` IN (SELECT U0.`object_id` FROM `uploads_image` U0 WHERE U0.`content_type_id` = 26 ) LIMIT 21
EDIT: czarchaic's suggestion has much nicer syntax but even worse (slower) performance. The SQL gen开发者_如何学编程erated by his query looks like the following:
SELECT DISTINCT `issues_article`.`id`, `issues_article`.`text`, COUNT(`uploads_image`.`id`) AS `num_images` FROM `issues_article` LEFT OUTER JOIN `uploads_image` ON (`issues_article`.`id` = `uploads_image`.`object_id`) GROUP BY `issues_article`.`id` HAVING COUNT(`uploads_image`.`id`) > 0 ORDER BY NULL LIMIT 21
EDIT: Hooray for Jarret Hardie! Here's the SQL generated by his should-have-been-obvious solution:
SELECT DISTINCT `issues_article`.`id`, `issues_article`.`text` FROM `issues_article` INNER JOIN `uploads_image` ON (`issues_article`.`id` = `uploads_image`.`object_id`) WHERE (`uploads_image`.`id` IS NOT NULL AND `uploads_image`.`content_type_id` = 26 ) LIMIT 21
Thanks to generic relations, you should be able to query this structure using traditional query-set semantics for reverse relations:
Article.objects.filter(images__isnull=False)
This will produce duplicates for any Article
s that are related to multiple Image
s, but you can eliminate that with the distinct()
QuerySet method:
Article.objects.distinct().filter(images__isnull=False)
I think your best bet would be to use aggregation
from django.db.models import Count
Article.objects.annotate(num_images=Count('images')).filter(num_images__gt=0)
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