text: from django.db import models
  
  # Create your models here.
  
  from django.urls import reverse #Used to generate urls by reversing the URL patterns
  
  class Genre(models.Model):
      """
      Model representing a book genre (e.g. Science Fiction, Non Fiction).
      """
      name = models.CharField(max_length=200, help_text="Enter a book genre (e.g. Science Fiction, French Poetry etc.)")
      
      def __str__(self):
          """
          String for representing the Model object (in Admin site etc.)
          """
          return self.name
  
  class Language(models.Model):
      """
      Model representing a Language (e.g. English, French, Japanese, etc.)
      """
      name = models.CharField(max_length=200, help_text="Enter a the book's natural language (e.g. English, French, Japanese etc.)")
      
      def __str__(self):
          """
          String for representing the Model object (in Admin site etc.)
          """
          return self.name
  
  
  class Book(models.Model):
      """
      Model representing a book (but not a specific copy of a book).
      """
      title = models.CharField(max_length=200)
      author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
        # 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 file.
      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>')
      genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
        # ManyToManyField used because Subject can contain many books. Books can cover many subjects.
        # Subject declared as an object because it has already been defined.
      language = models.ForeignKey('Language', on_delete=models.SET_NULL, null=True)
        
      def display_genre(self):
          """
          Creates 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'
      
      
      def get_absolute_url(self):
          """
          Returns the url to access a particular book instance.
          """
          return reverse('book-detail', args=[str(self.id)])
  
      def get_update_url(self):
          """
          Returns the url to access a particular book instance.
          """
          return reverse('book_update', args=[str(self.id)])
  
      def get_delete_url(self):
          """
          Returns the url to access a particular book instance.
          """
          return reverse('book_delete', args=[str(self.id)])
  
      def __str__(self):
          """
          String for representing the Model object.
          """
          return self.title
  
  import uuid # Required for unique book instances
  from datetime import date
  
  from django.contrib.auth.models import User #Required to assign User as a borrower
  
  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)
      
      @property
      def is_overdue(self):
          if self.due_back and date.today() > self.due_back:
              return True
          return False
          
  
      LOAN_STATUS = (
          ('d', 'Maintenance'),
          ('o', 'On loan'),
          ('a', 'Available'),
          ('r', 'Reserved'),
      )
  
      status= models.CharField(max_length=1, choices=LOAN_STATUS, blank=True, default='d', help_text='Book availability')
  
      class Meta:
          ordering = ["due_back"]
          permissions = (("can_mark_returned", "Set book as returned"),)   
  
      def __str__(self):
          """
          String for representing the Model object.
          """
          return '%s (%s)' % (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)
      
      def get_absolute_url(self):
          """
          Returns the url to access a particular author instance.
          """
          return reverse('author-detail', args=[str(self.id)])
  
      def get_update_url(self):
          """
          Returns the url to access a particular author instance.
          """
          return reverse('author_update', args=[str(self.id)])
  
      def get_delete_url(self):
          """
          Returns the url to access a particular author instance.
          """
          return reverse('author_delete', args=[str(self.id)])
      
  
      def __str__(self):
          """
          String for representing the Model object.
          """
          return '%s, %s' % (self.last_name, self.first_name)
  