class Cart(models.Model):
    user = models.OneToOneField(
        User,
        on_delete=models.CASCADE,
        related_name="cart",
        verbose_name=_("user"),
        null=True,
        blank=True,
        help_text=_("User is null for guest carts"),
    )
    session_key = models.CharField(
        _("session key"),
        max_length=40,
        blank=True,
        null=True,
        db_index=True,
        help_text=_("Session key for guest carts"),
    )
    created_at = models.DateTimeField(_("created at"), auto_now_add=True)
    updated_at = models.DateTimeField(_("updated at"), auto_now=True)

class Cart(models.Model):
    ....

    class Meta:
        ....
        constraints = [
            models.CheckConstraint(
                check=(
                    models.Q(user__isnull=False) | models.Q(session_key__isnull=False)
                ),
                name="cart_must_have_user_or_session",
            ),
        ]

user maydoni bo‘sh emas yoki session_key maydoni bo‘sh emas bo‘lishi kerak.

ALTER TABLE cart
ADD CONSTRAINT cart_must_have_user_or_session
CHECK (user_id IS NOT NULL OR session_key IS NOT NULL);