class Book(models.Model):
    title = models.CharField('Название', max_length=200)
    author = models.CharField('Автор', max_length=100)
    genre = models.CharField('Жанр', max_length=5, choices=get_categories())
    ...
    def __str__(self):
        return self.title

    def save(self, *args, **kwargs):
        self.pages = len(self.full_text.split()) / 20
        return

    def get_categories(self):
        return Categorie.getAll()


class Categorie(models.Model):
    category = models.CharField(max_length=100)

    # Return title for title in django admin
    def __str__(self):
        return self.category