Why is Loan undefined when type(self) works just fine?

Question

The following code is being used by the admin to save a Loan object

import uuid

from django.db import models
from django.contrib.auth.models import User
from apps.partners.models import Agent

# Create your models here.
class Loan(models.Model):
    """ This is our local info about the loan from the LOS """
    guid = models.CharField(max_length=64, blank=True)
    number = models.CharField(max_length=64, blank=True)
    name = models.CharField(max_length=64)
    address = models.CharField(max_length=128)
    address2 = models.CharField(max_length=128, null=True, blank=True)
    city = models.CharField(max_length=32)
    state = models.CharField(max_length=2)
    zipcode = models.CharField(max_length=9)
    borrowers = models.ManyToManyField(User, related_name='loan_borrowers')
    officers = models.ManyToManyField(Agent, related_name='loan_officers')

    def __unicode__(self):
        return "%s %s, %s  %s" % (self.address, self.city, self.state, self.zipcode)

    def save(self, force_insert=False, force_update=False, using=None):
        """ Adds a GUID if one is not present """
        if self.guid == None:
            self.guid = uuid.uuid4().hex
        super(Loan, self).save(force_insert, force_update, using)

When I get to the super line, I get:

TypeError: super() argument 1 must be type, not None

The save call is made from options.py line 597 and at that point obj is known to be a Loan object.

if I replace the super() line with

    super(type(self), self).save(force_insert, force_update, using)

all is well. What is going on here?

The rest of the file is:

class Need(models.Model):
    from apps.public_portal.models import DocumentType
    loan = models.ForeignKey(Loan, null=False, blank=False)
    borrower = models.ForeignKey(User, null=False, blank=False)
    doctype = models.ForeignKey(DocumentType, null=False, blank=False)
    satisfied = models.DateTimeField(null=True, blank=True)
    first_request = models.DateTimeField(auto_now_add=True)
    last_request = models.DateTimeField(null=True, blank=True)

    def __unicode__(self):
        return "%s from %s for %s" % (self.doctype.name, self.borrower.get_full_name(), self.loan)

So I don't see how anything is binding Loan to None

Answer 1

Something else is rebinding Loan to None. Look further down, or maybe in another module. Also, you don't want type(self) since that will fail for any grandchildren or further.

Answer 2

The Django developers offer a pattern for overriding the model save() method. Using that pattern, the implementation for your save() method should be:

def save(self, *args, **kwargs):
    if self.guid == None:
        self.guid = uuid.uuid4().hex
    super(Blog, self).save(*args, **kwargs) # Call the "real" save() method.

Let me offer a different approach: use signals!

Instead of trying to override the save() method, use a pre-save signal to initialize the guid field prior to the record being inserted/updated. Add the following code to your model.py file:

def add_loan_guid( sender, instance, **kwargs ):
    """Ensure that a Loan always has a guid"""
    if instance.guid == None:
        instance.guid = uuid.uuid4().hex

pre_save.connect( add_loan_guid, sender=Loan )

Now, any time that a Loan object is saved, but before it is persisted to the database, add_loan_guid() will be called, and if the Loan instance has no guid set, a new one will be created.