how to check a previous value on a django model

django, django-1.4, django-models, validationerror

Solution

model subclasses actually end up having a metaclass reformat all their class attributes. So you shouldn't be initializing a plain value as a class attribute. You should do it in the `__init__`

class MyModel(models.Model):
    version = models.FloatField()

    def __init__(self, *args, **kwargs):
        super(MyModel, self).__init__(*args, **kwargs)
        self._prev_value = self.version

    def clean(self):
        if self.version <= self._prev_value:
             raise ValidationError('error msg')

    def save(self,*args,**kwargs):
        super(MyModel, self).save(*args, **kwargs)
        self._prev_value = self.version

Problem

I've got a django model with a field that i want to be always greater than the previous one of the same model instance after an update, like this: ``` class MyModel(models.Model): version = models.FloatField() prev_value = 0 def clean(self): if self.version <= self.prev_value: raise ValidationError('error msg') def save(self,*args,**kwargs): super(MyModel, self).save(*args, **kwargs) self.prev_value = self.version ``` i know that clean is working fine because i've got other validation on the same methon and it's working just fine, what i'm doing wrong and how can i fix it ?. thanks in advance. I tested it and it didn't throws any error messages on updates with verion < prev_value Edit: im using new django 1.4

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