Django update table using data from another table

Question

I have 2 tables products and catagories connected by foreign key. I need to update field products.new_cost using field catagorie

Answer 1

rows = Product.objects.filter(old_field__isnull=False)
for row in rows:
     row.new_field = row.old_field.subfield
Product.objects.bulk_update(rows, ['new_field'])