Sorting Related objects in Django

Question

I have 2 models Category and Item. An Item has a reference to a Category.

class Category(models.Model):
    name = models.CharField(max_length=32)

class Item(model.Models):
    name = models.CharField(max_length=32)
    category = models.ForeignKey(Category)
    sequence = models.IntegerField()

The sequence field is supposed to capture the sequence of the Item within a category.

My question is: What Meta Options do i need to set on category and/or item such that when i do:

category.item_set.all()

that I get the Items sorted by their sequence number.

PS: I am now aware of a meta option called ordering_with_respect_to .. but it is still unclear how it works, and also i have legacy data in the sequence columns. I am open to data migration, if the right approach requires that.

Chris Pratt · Accepted Answer · 2011-05-23 16:20:08Z

11

What you're looking for is:

class Item(model.Models):
    name = models.CharField(max_length=32)
    category = models.ForeignKey(Category)
    sequence = models.IntegerField()

    class Meta:
        ordering = ['sequence',]

That will ensure that Items are always ordered by sequence.

answered May 23, 2011 at 16:20

Chris Pratt

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Uku Loskit · Accepted Answer · 2011-05-23 16:04:39Z

3

category.item_set.all().order_by('sequence')

edited May 23, 2011 at 16:04

answered May 23, 2011 at 15:31

Uku Loskit

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2 Comments

arustgi Over a year ago

Is there a way to set a default ordering so that I don't have to manually sequence it each time ?

arustgi Over a year ago

Thanks Uko. Would the order_with_respect_to that you have suggested, be on the Category model or the Item model?

Rob L · Accepted Answer · 2017-07-18 15:12:45Z

Kinda late, and the previous answers don't solve my specific question, but they led me to an answer, so I'm gonna throw this in:

I need to sort my prefetch_related objects specifically for only one view, so changing the default ordering is no good (maybe a model_manager would do it, idk). But I found this in the docs.

I have the following models:

class Event(models.Model):
    foo = models.CharField(max_length=256)
    ....

class Session(models.Model):
    bar = models.CharField(max_length=256)
    event = models.ForeignKey(Event)
    start = models.DateTimeField()
    ....

    class Meta:
        ordering = ['start']

Now in a particular view, I want to see all the Events, but I want their Sessions in reverse order, i.e., ordering = ['-start']

So I'm doing this in the view's get_queryset():

from django.db.models import Prefetch
session_sort = Session.objects.all().order_by('-start')
prefetch = Prefetch('sessions', queryset=session_sort)
events = Event.objects.all().prefetch_related(prefetch)

Hope it helps somebody!

*BTW, this is just a simplified version, there are a bunch of filters and other prefetch_related parameters in my actual use case.

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