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:

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

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.

yes use signal for post_save, when current_price will be changed according to it, the other 2 fields will be calculated and saved — Exprator
– Exprator, Commented Jul 5, 2017 at 7:43
@Exprator Oh yes, you mean this answer? stackoverflow.com/a/7934958/3371056 It seems good. — Milano
– Milano, Commented Jul 5, 2017 at 7:46
Although this is a good way, I'm thinking about using pre_save - is it possible? With pre_save, there would be just one commit. I'm not sure if I can access objects not commited changes in pre save. — Milano
– Milano, Commented Jul 5, 2017 at 7:59

Věroš K. · Accepted Answer · 2017-08-28 07:18:13Z

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)

QuerySets are lazy, using them as a boolean value will execute a corresponding query in the database. Instead remove the .first() and check the existence of the instance by doing old.exists()

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