Multi site news system: setup complexity problem

I am building a multi site news application. So, for instance: while creating a news item (article) I can select multiple sites where I want the item to be shown. Each site has his own "categories" and "dossiers".

Now, I have several models but I repeatedly questioning myself I this is the right approach.

Article Site Category Dossier (= file, not a physical file, but a file as in "part of larger collection of items around a certain subject". Kind of tag.)

An article is attached to 1 or more sites -> "ArticleSite" model

In my Article model I have this pivot relation defined:

public function sites()
{
	return $this->belongsToMany('App\Site')->withTimestamps()->using('App\ArticleSite');
}

The same for my Site model:

public function articles()
{
	return $this->hasMany('App\Article')->withTimestamps()->using('App\ArticleSite');
}

I created a ArticleSite model for the relationship.

class ArticleSite extends Pivot
{
	protected $table = 'article_sites';

	public function dossiers()
	{
		return $this->belongsToMany('App\Dossier', 'article_site_dossiers', 'articlesite_id', 'dossier_id');
	}
}

As you can see, I created a method "dossiers" on the ArticleSite model because a "dossier" is related to article+site. On site A it can be attached to dossier "X" while the same article on site B it can be attached to dossier "Y". Hence, this setup. I am also planning to do this for the categories. Each site holds different categories.

Now, I am having trouble with the dossiers() method. When saving a new article, I can the exception: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'articlesite_id' cannot be null (SQL: insert into article_site_dossiers (articlesite_id, dossier_id) values (?, 1), (?, 5))

This is the part of the ArticleController save method

$articleSite->save();
		
if (isset($request->input('dossiers')[$articleSiteId]))
{
	$articleSite->dossiers()->attach($request->input('dossiers')[$articleSiteId]);
}

(On the frontend I render multiple form sections for each selected site. That's why I use the brackets notation.)

I save the ArticleSite model, so there must be an Id available. I don't get it...

Am I trying to achieve the right setup, or am I trying to get something to work that shouldn't work in the first place? Are there other ways to achieve what I want?