Sure. In below is the Book and BookInstance model.py.
class Book(models.Model):
"""Model representing a book (but not a specific copy of a book)."""
title = models.CharField(max_length=200)
# Foreign Key used because book can only have one author, but authors can have multiple books
# Author as a string rather than object because it hasn't been declared yet in the file
author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
summary = models.TextField(max_length=1000, help_text='Enter a brief description of the book')
isbn = models.CharField('ISBN', max_length=13, help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
# ManyToManyField used because genre can contain many books. Books can cover many genres.
# Genre class has already been defined so we can specify the object above.
genre = models.ManyToManyField('Genre', help_text='Select a genre for this book')
language = models.ForeignKey('Language', on_delete=models.SET_NULL, null=True)
def __str__(self):
"""String for representing the Model object."""
return self.title
def get_absolute_url(self):
"""Returns the url to access a detail record for this book."""
return reverse('book-detail', args=[str(self.id)])
def display_genre(self):
"""Create a string for the Genre. This is required to display genre in Admin."""
return ', '.join(genre.name for genre in self.genre.all()[:3])
display_genre.short_description = 'Genre'
class BookInstance(models.Model):
"""Model representing a specific copy of a book (i.e. that can be borrowed from the library)."""
id = models.UUIDField(primary_key=True, default=uuid.uuid4, help_text='Unique ID for this particular book across whole library')
book = models.ForeignKey('Book', on_delete=models.SET_NULL, null=True)
imprint = models.CharField(max_length=200)
due_back = models.DateField(null=True, blank=True)
LOAN_STATUS = (
('m', 'Maintenance'),
('o', 'On loan'),
('a', 'Available'),
('r', 'Reserved'),
)
status = models.CharField(
max_length=1,
choices=LOAN_STATUS,
blank=True,
default='m',
help_text='Book availability',
)
class Meta:
ordering = ['due_back']
def __str__(self):
"""String for representing the Model object."""
return f'{self.id} ({self.book.title})'
In admin site i created a register of Book “test” and a register of BookInstance whose book field is that last Book “test”.
After that, i deleted that register of Book “test” and then, i cant remove or access the BookInstance.
In below is what appears when i try to acess the BookInstance (seen as " - ")
Environment:
Request Method: GET
Request URL: http://127.0.0.1:8000/admin/catalog/bookinstance/e4338198-f49a-4b94-b1bc-213fd61cca5d/change/
Django Version: 3.2.3
Python Version: 3.9.1
Installed Applications:
['django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'catalog.apps.CatalogConfig']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.common.CommonMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',
'django.middleware.clickjacking.XFrameOptionsMiddleware']
Traceback (most recent call last):
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\core\handlers\exception.py", line 47, in inner
response = get_response(request)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\core\handlers\base.py", line 181, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\contrib\admin\options.py", line 616, in wrapper
return self.admin_site.admin_view(view)(*args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\utils\decorators.py", line 130, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\views\decorators\cache.py", line 44, in _wrapped_view_func
response = view_func(request, *args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\contrib\admin\sites.py", line 232, in inner
return view(request, *args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\contrib\admin\options.py", line 1660, in change_view
return self.changeform_view(request, object_id, form_url, extra_context)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\utils\decorators.py", line 43, in _wrapper
return bound_method(*args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\utils\decorators.py", line 130, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\contrib\admin\options.py", line 1540, in changeform_view
return self._changeform_view(request, object_id, form_url, extra_context)
File "C:\Users\fabri\Envs\my_django_environment\lib\site-packages\django\contrib\admin\options.py", line 1632, in _changeform_view
'subtitle': str(obj) if obj else None,
File "C:\Users\fabri\Desktop\django\locallibrary\catalog\models.py", line 79, in __str__
return f'{self.id} ({self.book.title})'
Exception Type: AttributeError at /admin/catalog/bookinstance/e4338198-f49a-4b94-b1bc-213fd61cca5d/change/
Exception Value: 'NoneType' object has no attribute 'title'
