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 __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)
          return "{0} ({1})".format(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 "{0}, {1}".format(self.last_name, self.first_name)
  