Django: tracking if a field in the model is changed

Question

I have a model:

class Object(Object1): name = models.CharField(max_length=255, blank=True) description = models.TextField(blank=True) date_updated = models.DateTimeField(blank=True, null=True)

I want to track if there is anything inserted into the date_updated field, then create another object in another model, without overriding the save method.

For example:

if date_updated: MyModel.objects.create(type="D", user=request.user)

Although I have tried this, but still no success.

"without overriding the save method" - why do you have this requirement? — alecxe
– alecxe, Commented Apr 13, 2016 at 13:43
it is actually not a requirement, I just tried to override the save method, i also tried signals, and all the possible solutions in the link attached above, but no success. — arrt_
– arrt_, Commented Apr 13, 2016 at 13:46
Django signals like pre_save or post_save could be useful: docs.djangoproject.com/en/1.9/ref/signals — Shang Wang
– Shang Wang, Commented Apr 13, 2016 at 13:47

Ibrohim Ermatov · Accepted Answer · 2018-01-31 06:12:26Z

You can use tracker field from django-model-utils.

Add a tracker to the model:

class Object(Model): name = models.CharField(max_length=255, blank=True) description = models.TextField(blank=True) date_updated = models.DateTimeField(blank=True, null=True) tracker = FieldTracker()

You can check in save() or in other places where you usually update the model:

if object.tracker.has_changed('date_updated'): create_new_object(data)

C14L · Accepted Answer · 2016-04-13 14:03:13Z

Sorry, just noticed that you are referencing request.user. So this will NOT work for you. Since you need the particular request object, its probably best done in the view that the request object is referring to.

With pre_save you could compare the current instance's property value with the one currently in the database.

@receiver(pre_save, sender=MyModel) def check_date(sender, instance=None, created=False, **kwargs): changed = created or instance.date_updated != MyModel.objects.get(pk=instance.pk).date_updated if changed: MyModel.objects.create(type="D", user=request.user)

Didn't test it, though.

can you please give an example of how to do this in my case? because i tried a lot of things so far.
pre_save signal has not created arg, it is in post_save signal.

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