django: coercing to Unicode: need string or buffer, int found

Question

I'm getting this error when trying to sum up two int values:

if dups.count() > 0:
        for item in dups:
            pi.quantity = pi.quantity+item.quantity

both pi and dups are instance of same model:

class PurchaseItem(models.Model):
    picture = models.ForeignKey(Picture, null=False)
    paperType = models.ForeignKey(paperType, null=False)
    printSize = models.ForeignKey(printSize, null=False)
    quantity = models.IntegerField(default=1, validators=[validators.MinValueValidator(1)])
    price = models.DecimalField(decimal_places=2,max_digits=8)
    dateCreated = models.DateTimeField(null=False)
    sessionKey = models.ForeignKey(Session, to_field="session_key", null=False)
    user = models.ForeignKey(User,null=True)

    def __unicode__(self):
        return self.id 

Why is int not good enough?

if I wrap the values with str() or use their .str() representation, it doesn't do quite what I need. 1 and 1 will before 11 instead of 2.

Solution 1

Apparently, item.quantity is an integer, and pi.quantity is unicode.

I'm guessing pi.quantity was assigned a string previously (which django will convert upon db save), but it won't return the coerced value until you instantiate the class again. The value is cached according to some shell sessions.

Just do pi.quantity = int(pi.quantity) + item.quantity or look to where pi.quantity was defined and use integers instead!

Solution 2

Your code : def unicode(self): return self.id

Should be: def unicode(self): return unicode(self.id )