Laravel Eloquent and Mysql join a table IF another join is null

Three main tables:

products

advertisers

locations

Two pivot tables:

advertisers_locations

products_locations

Relationships:

A product belongs to an advertiser and an advertiser has many locations (Locations it can ship products to)

A product can also have it own set of locations that override the advertiser locations (Some products have delivery restrictions)

What I need to do is:

1. Select all products

2. Check if products_locations table for product ID and join it.

3. If it does not exist then join the advertisers locations table

Is this possible to do in one query and using eloquent? Here's my code - struggling with the conditional:

   public function scopeWhereShippableToLocation($query)
{
    $location_id = session('location_id');

    $query = $query->join('products_locations', function ($q) use ($location_id) {
        $q->on('products_locations.product_id', '=', 'products.id')
            ->where('products_locations.location_id', '=', $location_id);
    });

    $query = $query->join('advertisers_locations', function ($q) use ($location_id) {
        $q->on('advertisers_locations.advertiser_id', '=', 'products.advertiser_id')
            ->where('advertisers_locations.location_id', '=', $location_id);
    });

    return $query;
}

via Chebli Mohamed