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, Jul 05 '17 at 07:43
@Exprator Oh yes, you mean this answer? https://stackoverflow.com/a/7934958/3371056 It seems good. — Milano, Jul 05 '17 at 07:46
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, Jul 05 '17 at 07:59
its better to use in post_save as you will get the actual value as the signal will confirm the value has been saved — Exprator, Jul 05 '17 at 08:01

score 15 · Accepted Answer · edited Aug 28 '17 at 07:18

15

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)

edited Aug 28 '17 at 07:18

Věroš K.

103
3

answered Aug 26 '17 at 08:34

Milano

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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()` – Cyzanfar Apr 19 '22 at 14:08

Detect field change in Django model

1 Answers1