Django - 如何在保存之前获取字段值？

Question

I'm new with Django and I've been trying to get the value from a model field before I save it, for example:

class Student(models.Model):
    name = models.CharField(max_length=30, null=False)
    id = models.CharField(max_length=30, null=False, primary_key=True)
    phone = models.BigIntegerField(max_length=50, null=False)


def print_name():
    #here I want to catch and print the student's name before it saves

Answer 1

Just override the save method inside your model.

class Student(models.Model):
    name = models.CharField(max_length=30, null=False)
    id = models.CharField(max_length=30, null=False, primary_key=True)
    phone = models.BigIntegerField(max_length=50, null=False)

    def save(self):
        print self.name
        super(Student, self).save()

Answer 2

For some reason, none of the answers worked for me.

I was still able to solve the problem by retrieving the object from the database and comparing it against the version being saved:

def save(self, *args, **kwargs):
    old_object_instance = YourModel.objects.get(pk=self.pk)
    if old_object_instance.your_value != self.your_value:
        do_something_here()
    super().save(*args, **kwargs)

Answer 3

You can achieve that by overriding the default save method like this.

class Student(models.Model):
    name = models.CharField(max_length=30, null=False)
    id = models.CharField(max_length=30, null=False, primary_key=True)
    phone = models.BigIntegerField(max_length=50, null=False)
    def save(self, *args, **kwargs):
        print self.name
        super(Student, self).save(*args, **kwargs)

Answer 4

It does not really matter if it's in save() method or in any other method within the model class, you can access any model attribute with self which refers to the current instance:

class Student(models.Model):
    name = models.CharField(max_length=30, null=False)
    id = models.CharField(max_length=30, null=False, primary_key=True)
    phone = models.BigIntegerField(max_length=50, null=False)

    def any_function(self, any_arg1, any_arg2):
       ...  
       print self.name
       return self.name

Answer 5

Here i give you another solution using pre_save Signal

from django.db.models.signals import pre_save
from django.dispatch import receiver

@receiver(pre_save, sender=YourModel)
def my_callback(sender, instance, *args, **kwargs):
    print instance.name

Answer 6

Depending on your needs, previous answers could or could not work.

If you use this:

def save(self, *args, **kwargs):
    print(self.name)
    super(self).save(*args, **kwargs)

This only will work if your are editing a previously saved object. If you are creating a new object and want to get a field value before saving, you need to do this (will also work in case you are editing).

def save(self, *args, **kwargs):
    print(self._meta.get_field('name').value_from_object(self))
    super(Student, self).save(*args, **kwargs)