When saving, how can you check if a field has changed?

Question

In my model I have :

class Alias(MyBaseModel):
    remote_image = models.URLField(max_length=500, null=True, help_text=\"A URL that is downloaded and cached          


        

Answer 1

And now for direct answer: one way to check if the value for the field has changed is to fetch original data from database before saving instance. Consider this example:

class MyModel(models.Model):
    f1 = models.CharField(max_length=1)

    def save(self, *args, **kw):
        if self.pk is not None:
            orig = MyModel.objects.get(pk=self.pk)
            if orig.f1 != self.f1:
                print 'f1 changed'
        super(MyModel, self).save(*args, **kw)

The same thing applies when working with a form. You can detect it at the clean or save method of a ModelForm:

class MyModelForm(forms.ModelForm):

    def clean(self):
        cleaned_data = super(ProjectForm, self).clean()
        #if self.has_changed():  # new instance or existing updated (form has data to save)
        if self.instance.pk is not None:  # new instance only
            if self.instance.f1 != cleaned_data['f1']:
                print 'f1 changed'
        return cleaned_data

    class Meta:
        model = MyModel
        exclude = []