Django update table using data from another table

问题:

I have 2 tables products and catagories connected by foreign key. I need to update field products.new_cost using field catagories.price_markup as following:

UPDATE products p
INNER JOIN categories c ON p.category_id = c.id
SET p.new_cost = ROUND(p.pleer_cost * (1 + c.price_markup/100), -1)
WHERE p.update = 1

In SQL it's so easy, but how to do it using Django ORM?

My simplified try doesn't work Cannot resolve keyword 'category.price_markup' into field.:

Product.actived.select_related('category').filter(update=1)).update(new_cost=F('pleer_cost') * F('category.price_markup'))

回答1:

Django uses __ (double underscore) for related fields. Change that category.price_markup to a category__price_markup and you should be in the clear.

回答2:

Note: My answer is outdated now, Django 1.11 introduced OuterRef which implements this feature. Check Andrey Berenda answer below.

According to the documentation, updates using join clauses are not supported, see:

However, unlike F() objects in filter and exclude clauses, you can’t introduce joins when you use F() objects in an update – you can only reference fields local to the model being updated. If you attempt to introduce a join with an F() object, a FieldError will be raised:

# THIS WILL RAISE A FieldError
>>> Entry.objects.update(headline=F('blog__name'))

Also, according to this issue, this is by design and there is no plans to change it in the near future:

The actual issue here appears to be that joined F() clauses aren't permitted in update() statements. This is by design; support for joins in update() clauses was explicitly removed due to inherent complications in supporting them in the general case.

回答3:

You cannot use F, but you can use Subquery and OuterRef:

from django.db.models import Subquery, OuterRef

cost = Category.objects.filter(
    id=OuterRef('product_id')
).values_list(
    'price_markup'
)[:1]

Product.objects.update(
    new_cost=Subquery(cost)
)

回答4:

AFAIU it can be workarounded with

for row in ModelName.objects.filter(old_field__isnull=False):
     row.new_field = row.old_field.subfield
     row.save()