Detecting a change in model [duplicate]

Question

I am using Django for a web application and I want to implement some code to detect changes in my model (and use that to update the model). I have fiddled with save:

class MyModel(models.Model): def save(self, *args, **kw): if self.somevalue == 0: self.someothervalue = timezone.now() else: self.someothervalue = None super(MyModel, self).save(*args, **kw)

but this will also update someothervalue to timezone.now() when somevalue already was 0. I want it only to update if it becomes 0.

How do I do that?

@solarissmoke I checked that, but I don't seem to have the "old" instance, like I do with an update trigger in the database. — Bart Friederichs
– Bart Friederichs, Commented May 31, 2016 at 9:56

zxzak · Accepted Answer · 2016-05-31 10:37:52Z

Not tested but should work. You cache the previous value inside __init__ and detect if it has changed inside save.

class MyModel(models.Model): def __init__(self, *args, **kwargs): super(MyModel, self).__init__(*args, **kwargs) self.prev_somevalue = self.somevalue def save(self, *args, **kw): if self.somevalue == 0 and self.prev_somevalue != self.somevalue: self.someothervalue = timezone.now() else: self.someothervalue = None super(MyModel, self).save(*args, **kw)

Alternatively, if you are using forms you could do that on the ModelForm level by using the changed_data property.

Lastly, if you want to use signals, you should use the pre_save signal and retreive the previous from the database.

wobbily_col · Accepted Answer · 2016-05-31 15:50:56Z

I prefer this way of checking for changes in the save method:

class MyModel(models.Model) def save(self, *args, **kwargs): db_object = MyModel.objects.get(id=self.id) # this is the copy of the object saved in the database if self.somevalue == 0 and db_object.somevalue != 0: self.someothervalue = timezone.now() else: self.someothervalue = None super(MyModel, self).save(*args, **kwargs)

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