使用计算在 Django 中批量更新

Question

I have 2 models in my project:

class Currency(models.Model):
    title = models.CharField(max_length=100, unique=True)
    value = models.FloatField()

class Good(models.Model):
    name = models.CharField(max_length=100)
    slug = SlugField(max_length=100, unique=True)
    cost_to_display = models.IntegerField(default=0)
    cost_in_currency = models.IntegerField()
    currency = models.ForeignKey(Currency)

The idea of such model is to speed up the search by price and have all goods in one currency. Therefore I need some hook which will update all Goods in case exchange rate was updated.

In raw sql it will looks like this

mysql> update core_good set cost_to_display = cost_in_currency * (select core_currency.value from core_currency where core_currency.id = currency_id ) ;
Query OK, 663 rows affected (0.10 sec)
Rows matched: 7847  Changed: 663  Warnings: 0

Works pretty fast. Though I tried to implement the same in django admin like this (using bulk-update ):

def save_model(self, request, obj, form, change):
    """Update rate values"""
    goods = Good.objects.all()
    for good in goods:
        good.cost_to_display = good.cost_in_currency * good.currency.value
    bulk_update(goods)
    obj.save()

It takes up to 20 minutes to update all records via django admin this way.

What I am doing wrong? What is the right way to update all the prices?

Answer 1

This is purely untested, but it's sort of work in my mind:

from django.db.models import F
Good.objects.all().update(cost_to_display=F('cost_in_currenty') * F('currency__value'))

Even you are calling bulk_update , you still looped through all goods, which is why your process is slow.

Edit :

This won't work because F() doesn't support joined fields. It can be done using raw query.

Answer 2

For the future readers: any call to good.currency in your code is hitting the database. Consider using select_related to fetch Currency and Good objects in one query:

goods = Good.objects.select_related('currency')

Also now Django comes with bulk_update method since version 2.2 docs