[Answered ]-Django model class method using wrong `self`

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Well book has null=True, so self.book, can be None, and therefore self.book.title does not per se makes sense.

In the __str__ you thus will first have to check if there is indeed a linked book for that object, so:

class BookInstance(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    book = models.ForeignKey(Book, on_delete=models.RESTRICT, null=True)
    LOAN_STATUS = (
        ('m', 'Maintence'),
        ('o', 'On loan'),
        ('a', 'Available'),
        ('r', 'Reserved'),
    )

    status = models.CharField(
        max_length=1,
        choices=LOAN_STATUS,
        blank=True,
        default='m',
        help_text='Book Availability',
    )

    def __str__(self):
        if self.book_id:
            return f'{self.book.title} ({self.id})'
        return str(self.id)

It is however a bit odd to make this null=True, that thus means there can be BookInstances without a related Book? It might be better to clean the database and thus look for BookInstances with book=None, fix these, and mark the field as non-nullable.

👤willeM_ Van Onsem

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