CODEWITHSUNDEEP
pythondjangodjango-models
Sarius
Sarius

Reputation: 145

Django: From List of ForeignKey-Parents: get the on with the newest Child

is there a way to get, from a list of ForeignKey-Parents, the one which got the newest Child? For example:

class parent(models.Model):
    name = models.CharField(*)

class child(models.Model):
    parent = models.ForeignKey(parent, *)

Now, given a list of parents, i want to get the parent, which had a child added last. Is there an easy way to do that in Django?

Thanks for the Answers!

Upvotes: 1

Views: 505

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476574

You can not rely on the primary key to know what was last added. You better add a timestamp for this. So you might want to change your model to:

class Parent(models.Model):
    name = models.CharField(max_length=128)

class child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)
    created = models.DateTimeField(auto_now_add=True)

Then you can obtain the parent with the most recent child with:

Parent.objects.filter(pk__in=list_of_parents).order_by('-child__created').first()

This will return a Parent object if such parent exists, and None otherwise.

If adding a timestamp is not an option, you can try to work with the primary key, but a database does not per se distributes primary key values in a certain order:

# does not per se works

Parent.objects.filter(pk__in=list_of_parents).order_by('-child__pk').first()

Upvotes: 2

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