Saving model instance using pre_save signal

Question

These are my models:

class Stockdata(models.Model):
    user        = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete=models.CASCADE,null=True,blank=True,related_name='user_stock')
    company     = models.ForeignKey(Company,on_delete=models.CASCADE,null=True,blank=True)
    stock_name  = models.CharField(max_length=32)

class Stock_journal(models.Model):
    user                = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete=models.CASCADE,null=True,blank=True,related_name='user_closing')
    company             = models.ForeignKey(Company,on_delete=models.CASCADE,null=True,blank=True)
    stockitem           = models.OneToOneField(Stockdata,on_delete=models.CASCADE,null=True,blank=True,related_name='closingstock')
    closing_stock       = models.DecimalField(max_digits=10,decimal_places=2,null=True)

This is my signal:

@receiver(post_save, sender=Stockdata)
def create_default_stock_ledger(sender, instance, created, **kwargs):
    if created:
        Stock_journal.objects.create(user=instance.User,company=instance.Company,stockitem=instance)

I want to pass a pre_save signal of the same as I have done in my post_save signal i.e. I want to perform a pre_save signal function instead of a post_save signal..

When I try to do using pre_save signal I get the following error:

save() prohibited to prevent data loss due to unsaved related object 'stockitem'.

Any idea how to do this?

Thank you

Answer 1

You are assigning unsaved stockitem(Stockdata) object to a OneToOneField and thus it raises an error.

When you are assigning stockitem(Stockdata) object to OneToOneField, Id is not generated as you haven't saved stockitem object and thus as error says it will cause a data loss while saving Stock_journal model.