title: Unable to save product sku to django model using post_save signal
body: <p>Hello i am trying to save product sku to django model but it doesnt save anything and fails silently</p>
  
  <p>here is the models.py </p>
  
  <pre><code>class Product(models.Model):
      brand = models.ForeignKey(Brand , related_name='products')
      category = models.ForeignKey('Category', verbose_name='categories', related_name='products' , default='')
      title = models.CharField(max_length=500)
      SKU = models.CharField(max_length=100 , blank=True ,  unique=True)
  
  def product_post_save_receiver(sender,instance,*args,**kwargs):
      if instance.SKU is None:
          var1 = instance.title[:3]
          var2 = instance.brand.title[:3]
          var3 = var1+var2
          instance.SKU = var3
          instance.save()
  
  post_save.connect(product_post_save_receiver,sender=Product)
  </code></pre>
  
  <p>what can i do to solve the above problem
  <bR>
  thanks in advance</p>