class Book(models.Model):
    def get_categories():
        a = []
        b = Categorie.objects.all()
        for i in range(len(Categorie.objects.all())):
            a.append((str(b[i].category).lower(), str(b[i].ru_title)))
        return a

    get_default = '0'
    title = models.CharField('Название', max_length=200)
    author = models.CharField('Автор', max_length=100)
    genre = models.CharField('Жанр', max_length=25, choices=get_categories())