Question

# models.py
class Gallery(models.Model):
    images = models.ManyToManyField(Image, null=True, blank=True)

class Image(models.Model):
    image = models.ImageField()


# views.py
class GalleryIndex(ListView):
    model = Gallery

I need to get thumbnail for every gallery, which is it’s very first/last/whatever image.
How can I LIMIT image by 1 for any gallery as a custom attribute (to not override Gallery.images) without calling second SQL query?

Answer 1

I should’ve read docs better. Standard QuerySet API can’t handle such cases efficiently (annotate() generates GROUP BY clause for each of parent fields which is slow) so I come with extra() method and raw subquery.

class GalleryIndex(ListView):
    queryset = Gallery.objects.extra(select={'thumb':
        """
        SELECT "image"
        FROM "app_image", "app_gallery_images"
        WHERE (
            "app_gallery"."id" = "app_gallery_images"."gallery_id"
            AND "app_gallery_images"."image_id" = "app_image"."id"
        )
        AND "app_image"."image" IS NOT NULL
        LIMIT 1
        """
    })

This queryset do exactly what I wanted, since SorlImageField (and ImageField) needs only filename to represent thumbnail in templates.