from django.db import models
from django.utils.text import compress_string
from django.db.models import signals
from django.dispatch import dispatcher

def uncompress_string(s):
    '''helper function to reverse django.utils.text.compress_string'''
    import cStringIO, gzip
    try:
        zbuf = cStringIO.StringIO(s)
        zfile = gzip.GzipFile(fileobj=zbuf)
        ret = zfile.read()
        zfile.close()
    except:
        ret = s
    return ret

class CompressedTextField(models.TextField):
    '''transparently compress data before hitting the db and uncompress after fetching'''

    def get_db_prep_save(self, value):
        if value is not None:
            value = compress_string(value) 
        return models.TextField.get_db_prep_save(self, value)
 
    def _get_val_from_obj(self, obj):
        if obj:
            return uncompress_string(getattr(obj, self.attname))
        else:
            return self.get_default() 
    
    def post_init(self, instance=None):
        value = self._get_val_from_obj(instance)
        if value:
            setattr(instance, self.attname, value)

    def contribute_to_class(self, cls, name):
        super(CompressedTextField, self).contribute_to_class(cls, name)
        dispatcher.connect(self.post_init, signal=signals.post_init, sender=cls)
    
    def get_internal_type(self):
        return "TextField"
                
    def db_type(self):
        from django.conf import settings
        if settings.DATABASE_ENGINE == 'mysql':
            return 'longblob'
        else:
            raise Exception, '%s currently works only with MySQL'%self.__class__.__name__


class Item(models.Model):
    title = models.CharField(maxlength=200)
    summary = CompressedTextField(blank=True)