Django 1.2: How to connect pre_save signal to class method

Question

I am trying to define a "before_save" method in certain classes in my django 1.2 project. I'm having trouble connecting the signal to the class method in models.py.

class MyClass(models.Model):
    ....
    def before_save(self, sender, instance, *args, **kwargs):
        self.test_field = "It worked"

I've tried putting pre_save.connect(before_save, sender='self') in 'MyClass' itself, but nothing happens.

I've also tried putting it at the bottom of the models.py file:

pre_save.connect(MyClass.before_save, sender=MyClass)

I read about connecting signals to class methods here, but can't figure out the code.

Anybody know what I'm doing wrong?

Answer 1

Rather than use a method on MyClass, you should just use a function. Something like:

def before_save(sender, instance, *args, **kwargs):
    instance.test_field = "It worked"

pre_save.connect(before_save, sender=MyClass)

Answer 2

A working example with classmethod:

class MyClass(models.Model):
    #....
    @classmethod
    def before_save(cls, sender, instance, *args, **kwargs):
        instance.test_field = "It worked"

pre_save.connect(MyClass.before_save, sender=MyClass)

There's also a great decorator to handle signal connections automatically: http://djangosnippets.org/snippets/2124/

Answer 3

I know this question is old, but I was looking for an answer to this earlier today so why not. It seems from your code that you actually wanted to use an instance method (from the self and the field assignment). DataGreed addressed how to use it for a class method, and using signals with instance methods is pretty similar.

class MyClass(models.Model)

    test_field = models.Charfield(max_length=100)

    def __init__(self, *args, **kwargs):
        super(MyClass, self).__init__(*args, **kwargs)
        pre_save.connect(self.before_save, sender=MyClass)

    def before_save(self, sender, instance, *args, **kwargs):
        self.test_field = "It worked"

I'm not sure if this is a good idea or not, but it was helpful when I needed an instance method called on an object of class A before save from class B.