Creating a conditional Many to Many relation in Django

It seems easy but I still can't figure out how to implement it.

Trying to create a Many to Many conditional relations (not sure if it's the correct way to name it!) between three models:

Class Book(models.Model):
    book_name = models.CharField(max_length=50)


Class Article(models.Model):
    article_name = models.CharField(max_length=50)


Class Portfolio(models.Model):
    KINDS = [('B', 'Book'), ('A', 'Article')]
    book = models.ForeignKey(Book, on_delete=models.PROTECT)
    article = models.ForeignKey(Article, on_delete=models.PROTECT)
    kind = models.models.CharField(max_length=1, choices=KINDS)
    user = models.ManyToManyField(User, through='Recommendation')


Class Recommendation(models.Model):
    user = models.ForeignKey(User, on_delete=models.PROTECT)
    portfolio = models.ForeignKey(Book, on_delete=models.PROTECT)
    date = models.DateTimeField()

Each User (from the Django User module) can have multiple recommendations of portfolios. Each Portfolio is a group of books and articles recommended on a specific date.

I would skip the Portfolio Model and just implement it this way:

class Book(models.Model):
    book_name = models.CharField(max_length=50)


class Article(models.Model):
    article_name = models.CharField(max_length=50)


class Recommendation(models.Model):
    user = models.ForeignKey(User, on_delete=models.PROTECT)
    book = models.ForeignKey(
        Book,
        blank=True,
        null=True,
        on_delete=models.PROTECT
    )
    article = models.ForeignKey(
        Article,
        blank=True,
        null=True,
        on_delete=models.PROTECT
    )
    date = models.DateTimeField()

    class Meta:
        constraints = [
            models.CheckConstraint(
                check=Q(book__isnull=True, article__isnull=False) | Q(book__isnull=False, article__isnull=True),
                name='only_book_or_article'
            )
        ]

you could skip the constrains but than you risk that your data is inconsistent