Django snippets

try:
    import json
except ImportError:
    from django.utils import simplejson as json

from django.db import models

class JsonDescriptor(property):
    def __init__(self, field):
        self.field = field
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        if self.field.name not in instance.__dict__:
            raw_data = getattr(instance, self.field.attname)
            instance.__dict__[self.field.name] = self.field.unpack(raw_data)
        return instance.__dict__[self.field.name]
    
    def __set__(self, instance, value):
        instance.__dict__[self.field.name] = value
        setattr(instance, self.field.attname, self.field.pack(value))


class JsonObjectField(models.TextField):
    """This is a specialized incarnation of the Django TextField, designed
    to transparently pack/unpack objects into JSON representations.
    
    """
    def pack(self, obj):
        return json.dumps(obj)
    
    def unpack(self, data):
        return json.loads(str(data))
    
    def get_attname(self):
        return '%s_json' % self.name
    
    def contribute_to_class(self, cls, name):
        super(JsonObjectField, self).contribute_to_class(cls, name)
        setattr(cls, name, JsonDescriptor(self))