Django ORM m2m limit to 1 - django

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

A many-to-many acts as a descriptor for a normal queryset, so you can do my_gallery.images.all()[0] to limit the query to 1.

Answer 2

I don't think I understand correctly what you want to do, but doesn't below code work for you?
class Gallery(models.Model):
images = models.ManyToManyField(Image, null=True, blank=True)
def get_thumb(self):
return self.images.all()[0]
Or maybe other concept:
class Gallery(models.Model):
images = models.ManyToManyField(Image, null=True, blank=True)
thumbnail = models.ImageField()
def save(self, *args, **kwargs):
self.thumbnail = self.images.all()[0].image
(some exception catching here needed though)

Answer 3

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.