[Solved]-Django: how to get Foreign key id?

You can get the “raw” value of any foreign key in Django by adding “_id” to the field name

obj = ProductImage.objects.get()
obj.product_id  # Will return the id of the related product

You can also just follow the relationship but this will perform another DB lookup if the relationship has not been cached by using something like select_related

obj.product.id

Here is what I have tried so far and found the solution. The only option I found for achieving is using pre_save and post_save signals. And here is how I achieved the solution. If anyone has different solution, please share. Thanks.

from django.db.models.signals import post_save, pre_save
from django.dispatch import receiver

_UNSAVED_IMAGEFIELD = 'unsaved_imagefield'

def upload_path_handler(instance, filename):
    import os.path
    fn, ext = os.path.splitext(filename)
    return "images/{id}/{fname}".format(id=instance.product_id, 
    fname=filename)

class ProductImage(models.Model):
   product = models.ForeignKey(Product, on_delete=models.DO_NOTHING)
   image = models.ImageField(upload_to=upload_path_handler, blank=True)

@receiver(pre_save, sender=ProductImage)
def skip_saving_file(sender, instance, **kwargs):
    if not instance.pk and not hasattr(instance, _UNSAVED_IMAGEFIELD):
        setattr(instance, _UNSAVED_IMAGEFIELD, instance.image)
        instance.image = None

@receiver(post_save, sender=ProductImage)
def update_file_url(sender, instance, created, **kwargs):
    if created and hasattr(instance, _UNSAVED_IMAGEFIELD):
        instance.image = getattr(instance, _UNSAVED_IMAGEFIELD)
        instance.save()

Just add str function in your foreign reference model Product.

class Product(models.Model):
  product_name = models.CharField(max_length=100)
  product_weight = models.CharField(max_length=30)

  def __str__(self):
      return str(self.id)