Attribute Error: 'NoneType' object has no attribute 'id'

[imsanjoykb]

def str(self):
"""
String for representing the Model object
"""
if self.book:
return '%s (%s)' %(self.id, self.book.title))
else:
return '%s (%s)' %(self.id, self.imprint)) # add some other value that you want here