text: from django.db import models
  
  # Create your models here.
  
  class Genre(models.Model):
      """Model representing a book genre."""
      name = models.CharField(max_length=200, help_text='Enter a book genre (e.g. Science Fiction)')
  
      def __str__(self):
          """String for representing the Model object."""
          return self.name
  
  class Language(models.Model):
      """Model representing a book language."""
      name = models.CharField(max_length=2, help_text='Enter a book language (e.g en)')
  
      def __str__(self):
          """String for representing the Model object."""
          return self.name
  
  from django.urls import reverse # Used to generate URLs by reversing the URL patterns
  
  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])
  
      def book_detail_view(request, primary_key):
          try:
              book = Book.objects.get(pk=primary_key)
          except Book.DoesNotExist:
              raise Http404('Book does not exist')
          return render(request, 'catalog/book_detail.html', context={'book': book})
  
      display_genre.short_description = 'Genre'
  
  import uuid # Required for unique book instances
  from django.contrib.auth.models import User
  from datetime import date
  
  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)
      borrower = models.ForeignKey(User, on_delete=models.SET_NULL, 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',
                                )
      @property
      def is_overdue(self):
          if self.due_back and date.today() > self.due_back:
              return True
          return False
  
      class Meta:
         ordering = ['due_back']
         permissions = (("can_mark_returned", "Set book as returned"),)
  
      def __str__(self):
          """String for representing the Model object."""
          return f'{self.id} ({self.book.title})'
  
  class Author(models.Model):
      """Model representing an author."""
      first_name = models.CharField(max_length=100)
      last_name = models.CharField(max_length=100)
      date_of_birth = models.DateField(null=True, blank=True)
      date_of_death = models.DateField('Died', null=True, blank=True)
  
      class Meta:
          ordering = ['last_name', 'first_name']
  
      def get_absolute_url(self):
          """Returns the url to access a particular author instance."""
          return reverse('author-detail', args=[str(self.id)])
  
      def __str__(self):
          """String for representing the Model object."""
          return f'{self.last_name}, {self.first_name}'
  