Copying Django model row data before updating

问题

Before updating a row, I'd like to save its current results into another table. Currently I am using pre_save but it does not seem to work as intended. It gives me updated data, not pre-updated data.

class Country(models.Model):
    name = models.CharField(max_length=16)

def make_copy(sender, **kwargs):
    obj = kwargs['instance']

pre_save.connect(make_copy, sender=Country)

If I change the country name from USA to Australia, for example, the obj.name in pdb will display the post-updated name of Australia rather than pre-updated name USA.

What am I missing?

回答1:

you would have to query the database to get the original object. instance has the updated object, which is ready to be saved into the database.

class Country(models.Model):
    name = models.CharField(max_length=16)

def make_copy(sender, **kwargs):
    obj = kwargs['instance']
    try:
        orig_obj = Country.objects.get(pk=obj.pk)
    except: #If it is a new object
        orig_obj = None

pre_save.connect(make_copy, sender=Country)

回答2:

The previous data only exists in the database; you will need to retrieve it in your handler.

来源：https://stackoverflow.com/questions/17076932/copying-django-model-row-data-before-updating