Foreign key validation in model cleanup method - Django 1.7.7 vs 1.8

I have some code in the clean () method that works fine on Django 1.7.7, but stops working in 1.8

Scenario

class Span(models.Model):
    happening = models.ForeignKey('Happening', blank = True, null = True)
    soul_class = models.ForeignKey('SoulClass', blank = True, null = True)

      

        
        
        
      

    

In a clean()

class method Span

, I have a check as follows

if self.happening:
    foo

if self.soul_class:
    bar

      

        
        
        
      

    

This worked fine in Django up to 1.7.7, but when I upgrade to Django 1.8, both self.happening and self.soul_class are None.

The main difference seems to be the meaning _soul_class_cache

as follows:

The Span object in 1.7.7 looks like

{'end_date': None,
 'id': None, 
 '_state': `<`django.db.models.base.ModelState object at 0x7ff046ac1320`>`, 
 'start_date': datetime.date(2015, 4, 16), 
 'soul_class_id': None, 
 'end_time': datetime.time(14, 12), 
 'happening_id': None, 
 '_happening_cache': None, 
 'start_time': datetime.time(14, 12), 
 '_soul_class_cache': `<`SoulClass: Chi Kung`>` # <-- Here
}

      

        
        
        
      

    

In 1.8

{'start_time': datetime.time(14, 0), 
 'end_date': None, 
 'soul_class_id': None, 
 '_state': `<`django.db.models.base.ModelState object at 0x7f88d698d2e8`>`, 
 'happening_id': None, 
 '_happening_cache': None, 
 '_soul_class_cache': None, # <-- Here 
 'end_time': datetime.time(15, 0), 
 'id': None,
 'start_date': datetime.date(2015, 4, 16)}

      

        
        
        
      

    

Any ideas?