Django: denormalization in models vs. raw sql

models.py

from django.db import models
from django.db.models import signals
from django.utils.translation import ugettext_lazy as _

from .signals import update_thumbnail_image


class Gallery(models.Model):

    title = models.CharField(_('title'), max_length=50)
    thumbnail = models.ImageField(_('thumbnail'),
                                  upload_to='galleries/gallery',
                                  editable=False)
    images_count = models.PositiveIntegerField(_('images count'),
                                               editable=False,
                                               default=0)

    class Meta:
        ordering = ('-images_count',)
        verbose_name = _('gallery')
        verbose_name_plural = _('galleries')

    def __unicode__(self):
        return self.title

    def update_images_count(self):
        self.images_count = self.images.count()
        self.save()
    update_images_count.alters_data = True


class Image(models.Model):

    gallery = models.ForeignKey('galleries.Gallery', related_name='images')
    title = models.CharField(_('title'), max_length=50)
    image = models.ImageField(_('image'), upload_to='galleries/image')

    class Meta:
        verbose_name = _('image')
        verbose_name_plural = _('images')

    def __unicode__(self):
        return self.title

    def save(self, *args, **kwargs):
        super(Image, self).save(*args, **kwargs)
        self.gallery.update_images_count()


signals.post_save.connect(update_thumbnail_image, sender=Image)

signals.py

from django.db.models import ImageField


def update_thumbnail_image(instance, **kwargs):
    if kwargs.get('raw', False):
        return
    # могу ошибаться в строчке ниже, вроде обязательно надо save=False
    instance.gallery.thumbnail = ImageField(instance.image, save=False)
    instance.gallery.save()