How to check if a particular field has changed when saving an instance?

Question

Django

There is this model:

from django.db import models
class Car(models.Model)
    price = models.IntegerField('Price')

When saving an instance of this model, I need to check how the Price field has changed (if it has changed at all). If the price has become more, then do one thing, if less, then do another. Something like this:

from django.db.models.signals import post_save
@receiver(post_save, sender=Car)
def car_is_saved(instance, **kwargs):
    if instance.newprice > instance.price:
        # Цена увеличилась
    elif instance.newprice < instance.price:
        # Цена уменьшилась
    else:
        # Цена не изменилась

Where to get instance.newprice?

Answer 1

I found such a solution, but it creates an additional query to the database. I would like to avoid this, of course.

@receiver(pre_save, sender=Car)
def car_is_saved(sender, instance, **kwargs):
    try:
        obj = sender.objects.get(pk=instance.pk)
    except sender.DoesNotExist:
        pass
    else:
        if obj.price == instance.price:
            # Цена не изменилась
            pass
        elif obj.price < instance.price:
            # Цена поднялась
        elif obj.price > instance.price:
            # Цена снизилась

Answer 2

Да, официальный - www.woothemes.com/products/product-vendors
Могу продать, у меня он есть