How can I get previous and next objects from a filtered, ordered queryset?

Question

I have a page based on a model object, and I want to have links to the previous and next pages. I don\'t like my current solution because it requires evaluating the entire query

Answer 1

I'm new to Python and Django, so maybe my code is not optimal, but check this out:

def get_prev_and_next_items(target, items):
    ''' To get previous and next objects from QuerySet '''
    found = False
    prev = None
    next = None
    for item in items:
        if found:
            next = item
            break
        if item.id == target.id:
            found = True
            continue
        prev = item
    return (prev, next)

And in view something like that:

def organisation(request, organisation_id):
    organisation = Organisation.objects.get(id=organisation_id)
    ...
    prev, next = get_prev_and_next_items(organisation, Organisation.objects.all().order_by('type'))
    ...
    return render_to_response('reference/organisation/organisation.html', {
        'organisation': organisation,
        'prev': prev,
        'next': next,
    })

Definitely not optimal for «heavy» querysets, but in most cases works like a charm. :)

Answer 2

Sounds like something the Paginator set to a threshold of 1 would do well at.

# Full query set...
pages = Page.objects.filter(column=somevalue)
p = Paginator(pages, 1)

# Where next page would be something like...
if p.has_next():
     p.page(p.number+1)

Documentation here and here.

Answer 3

Check out django-next-prev, I wrote it to solve this exact problem. In this case:

from next_prev import next_in_order, prev_in_order

def get_prev_and_next_page(current_page):
    page_qs = Page.objects.all()  # this could be filtered, or ordered
    prev_page = prev_in_order(current_page, page_qs)
    next_page = next_in_order(current_page, page_qs)
    return (prev_page, next_page)