Django - How to update a parent model using a child model?

I have a two models something like this:

models.py

class Parent(models.Model):
  id = models.BigAutoField(primary_key=True)
  quantity = models.DecimalField(max_digits=3, decimal_places=0, null=True)

class Child(models.Model):
  parent = models.ForeignKey('Parent', related_name="parent_child", on_delete=models.CASCADE)
  quantity = models.DecimalField(max_digits=3, decimal_places=0, null=True)

If the parent quantity is different from the sum of the child quantities, Updates the parent quantity to the sum of the child quantities.

I tried to get an object whose parent quantity is different from the sum of the child quantities:

parent = Parents.objects.annotate(sum_quantities=Sum('parent_child__quantity')).exclude(quantity=F('sum_quantities'))

It works! But I'm having trouble updating this. Is there a good way?

Yes, you can use the update() method of the queryset to update the parent quantity to the sum of the child quantities:

Parent.objects.annotate(sum_quantities=Sum('parent_child__quantity')).filter(quantity__ne=F('sum_quantities')).update(quantity=F('sum_quantities'))

This will update the quantity field of all the parents whose quantity is different from the sum of quantity fields of their children.

I solved it.

Parent.objects.annotate(
    sum_quantities=Sum('parent_child__quantity')).exclude(quantity=F('sum_quantities')).update(
        quantity=Subquery(
            Parent.objects.filter(parent=OuterRef('id')).annotate(sum_quantities=Sum('parent_child__quantity')).values('sum_quantities')[:1]    
        )
    )
)
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