Django snippets

from django.db import models
from django.conf import settings
from django.core import serializers
from django.db.models.query import QuerySet


class SerializedObjectField(models.TextField):
    '''Model field that stores serialized value of model class instance
       and returns deserialized model instance
       
       >>> from django.db import models
       >>> import SerializedObjectField
       
       >>> class A(models.Model):
               object = SerializedObjectField(serialize_format='json')
               
       >>> class B(models.Model):
               field = models.CharField(max_length=10)
       >>> b = B(field='test')
       >>> b.save()
       >>> a = A()
       >>> a.object = b
       >>> a.save()
       >>> a = A.object.get(pk=1)
       >>> a.object
       <B: B object>
       >>> a.object.__dict__
       {'field': 'test', 'id': 1}
       
    '''
    def __init__(self, serialize_format='json', *args, **kwargs):
        self.serialize_format = serialize_format
        super(SerializedObjectField, self).__init__(*args, **kwargs)
        
    def _serialize(self, value):
        if not value:
            return ''
        
        if not isinstance(value, QuerySet):
            value = [value]
            
        return serializers.serialize(self.serialize_format, value)
 
    def _deserialize(self, value):
        objs = [obj for obj in serializers.deserialize(self.serialize_format,
                                value.encode(settings.DEFAULT_CHARSET))]
        
        if len(objs) == 1:
            return objs[0].object
        else:
            return [obj.object for obj in objs]
 
    def db_type(self):
        return 'text'
 
    def pre_save(self, model_instance, add):
        value = getattr(model_instance, self.attname, None)
        return self._serialize(value)
 
    def contribute_to_class(self, cls, name):
        self.class_name = cls
        super(SerializedObjectField, self).contribute_to_class(cls, name)
        models.signals.post_init.connect(self.post_init)
 
    def post_init(self, **kwargs):
        if 'sender' in kwargs and 'instance' in kwargs:
            if kwargs['sender'] == self.class_name and \
            hasattr(kwargs['instance'], self.attname):
                value = self.value_from_object(kwargs['instance'])
                
                if value:
                    setattr(kwargs['instance'], self.attname,
                            self._deserialize(value))
                else:
                    setattr(kwargs['instance'], self.attname, None)