Get all rows except any models already related

So I have 2 models in Laravel 5.3. Practice and Offer, they have a many to many relationship which is set up and working. I can attach offers to practices etc. I want to retrieve all offers that are not already related to the practice.

The logged in user also belongs to the practice with practice_id.

In my controller I am passing in $offer in the constructor which just relates to all offers. I've tried using the reject() method as below but I'm getting nothing back, and I can't figure it out.

If I use $this->offer I get everything.

$query = $this->offer->reject(function($offer){
    return $offer->whereHas('practices', function($q) {
        return $q->where('practice_id', Auth::user()->practice_id);
    });
});

Please help.

via Chebli Mohamed