Detect field change in Django model

Question

Is it possible to detect that field in Django model has been changed?

class Product(models.Model):
    my_current_price = MoneyField(max_digits=20, decimal_places=2, null=True, blank=True,
                                  verbose_name=_('My original price'))
    my_eur_price = MoneyField(max_digits=20, decimal_places=2, null=True, blank=True, verbose_name=_('My price in EUR'))
    my_usd_price = MoneyField(max_digits=20, decimal_places=2, null=True, blank=True, verbose_name=_('My price in USD'))

The thing is that I need to recalculate EUR and USD price anytime when my_current_price is changed.

I have two ideas:

1. Somehow override MoneyField and send signal when the field is changed.

2. Override save method and create a new attribute __my_current_price like here - this works but it makes code very unclear for me.

EDIT: I store the price in different currencies because of faster database lookups.

Answer 1

One way is to create a signal and compare instance with an object from database. But it seems that better way is to override save method which is a best practice.

def save(self,*args,**kwargs):
    old = Model.objects.filter(pk=getattr(self,pk,None)).first()
    if old:
        if old.attr!=self.attr:
            # attr changed
    super(Model,self).save(*args,**kwargs)