Wednesday, November 28

Ordering related objects in Django

Django models have a meta option order_with_respect_to that allows you to order objects within the scope of a related ForeignKey object.

This option adds an _order column to the model's database table to keep track of this ordering.

That's all very well, but if you need to change the sequence order you may well feel at a loss. The official documentation makes no mention of this option beyond a basic explanation of its purpose , and editing your objects in the built in admin app reveals no user interface for changing the ordering either. [1]

Update Feb 2009: There's now a ticket for this functionality to be added to the docs.

I managed to uncover a Python interface by delving into Django's API internals [2]. An object with related models that order with respect to it is given two handy methods:


where RELATED is the lowercased model name of the ordered objects

class Entry(model.Model):
# ... fields

class Comment(model.Model):
entry = model.ForeignKey(Entry, related_name='comments')
# ... more fields

class Meta:
order_with_respect_to = 'entry'

Given the above example models, you can retrieve the order of an entry's comments:

>>> entry = Entry.objects.latest()
>>> entry.get_comment_order()
[1, 2, 3, 4]

And change the ordering by passing a list of comment ids back in.

>>> entry.set_comment_order([2, 1, 4, 3])

N.B. Be sure to pass in the same ids returned by get_comment_order

Two other handy methods exist for the Comment objects, get_next_in_order and get_previous_in_order [3]

>>> comment = Comment.objects.get(pk=3)
>>> comment.get_next_in_order()
<Comment: 4>
>>> comment.get_previous_in_order()
<Comment: 2>

  1. A very early ticket is still open to restore this functionality to the admin app, and recent activity suggests it may well reappear soon. See also this thread on the django-developers mailing list. back

  2. Look particularly at the two helper functions method_set_order and method_get_order in django.db.models.base. back

  3. Theoretically at least, there's still a bug in get_previous_in_order but it should be fixed soon. Update: Fixed by the queryset-refactor back

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