Say I have a list of photos ordered by creation date, as follows:
class Photo(models.Model):
title = models.Char()
image = models.Image()
created = models.DateTimeField(auto_now_add=True)
class Meta:
ordering = ('-created',)
I have an arbitrary Photo object photo_x. Is there an easy way to find the previous and next photos by position in the queryset? Also, I would like to wrap around if I am at the beginning/end and have it not fail if the are only 1 or 2 photos.
user "q = Salaries. objects. all(). order_by('-salary)[1]" .. it will give you second highest salary record..
You use a period (.) to access the fields in the Django ORM object.
The Django ORM is a very powerful tool, and one of the great attractions of Django. It makes writing simple queries trivial, and does a great job of abstracting away the database layer in your application. And sometimes, you shouldn't use it.
You're in luck! Django creates get_next_by_foo
and get_previous_by_foo
methods by default for DateField
& DateTimeField
as long as they do not have null=True
.
For example:
>>> from foo.models import Request >>> r = Request.objects.get(id=1) >>> r.get_next_by_created() <Request: xyz246>
And if you reach the end of a set it will raise a DoesNotExist
exception, which you could easily use as a trigger to return to the beginning of the set:
>>> r2 = r.get_next_by_created() >>> r2.get_next_by_created() ... DoesNotExist: Request matching query does not exist.
Further reading: Extra instance methods
get_next_by_foo and get_previous_by_foo are handy, but very limited - they don't help you if you're ordering on more than one field, or a non-date field.
I wrote django-next-prev as a more generic implementation of the same idea. In your case you could just do this, since you've set the ordering in your Meta:
from next_prev import next_in_order, prev_in_order from .models import Photo photo = Photo.objects.get(...) next = next_in_order(photo) prev = prev_in_order(photo)
If you wanted to order on some other combination of fields, just pass the queryset:
photos = Photo.objects.order_by('title') photo = photos.get(...) next = next_in_order(photo, qs=photos)
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