Jason Christa Jason Christa - 3 months ago 13
Python Question

Is there a clever way to get the previous/next item using the Django ORM?

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.

Answer

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

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