How to structure a relationship between one model and two others

To handle the relationship between shipments and multiple carriers, you can use a polymorphic relationship. This allows a model to belong to more than one other model on a single association. In your case, both company locations and carriers can be considered as "Stoppable" entities (i.e., places where the shipment can stop).

Here's how you can structure your models and database tables:

1. Create a stoppables table that will act as a polymorphic relation. This table will store the type of stoppable (company or carrier) and its ID.

Schema::create('stoppables', function (Blueprint $table) {
    $table->id();
    $table->morphs('stoppable'); // This will add stoppable_type and stoppable_id columns
    $table->timestamps();
});

1. Create a shipments table that will reference the stoppables table for origin and destination.

Schema::create('shipments', function (Blueprint $table) {
    $table->id();
    $table->unsignedBigInteger('origin_id');
    $table->unsignedBigInteger('destination_id');
    $table->timestamps();

    $table->foreign('origin_id')->references('id')->on('stoppables');
    $table->foreign('destination_id')->references('id')->on('stoppables');
});

1. Define the polymorphic relationship in your Eloquent models.

class Shipment extends Model
{
    public function origin()
    {
        return $this->belongsTo(Stoppable::class, 'origin_id');
    }

    public function destination()
    {
        return $this->belongsTo(Stoppable::class, 'destination_id');
    }
}

class Stoppable extends Model
{
    public function stoppable()
    {
        return $this->morphTo();
    }
}

class CompanyLocation extends Model
{
    public function stoppables()
    {
        return $this->morphMany(Stoppable::class, 'stoppable');
    }
}

class Carrier extends Model
{
    public function stoppables()
    {
        return $this->morphMany(Stoppable::class, 'stoppable');
    }
}

1. When creating a shipment, you would first create a Stoppable record for both the origin and destination, setting the stoppable_type and stoppable_id to the appropriate values.

// Assuming you have a company location and a carrier instance
$companyLocation = CompanyLocation::find(1);
$carrier = Carrier::find(1);

// Create stoppables for origin and destination
$originStoppable = Stoppable::create([
    'stoppable_type' => get_class($companyLocation),
    'stoppable_id' => $companyLocation->id,
]);

$destinationStoppable = Stoppable::create([
    'stoppable_type' => get_class($carrier),
    'stoppable_id' => $carrier->id,
]);

// Create the shipment
$shipment = Shipment::create([
    'origin_id' => $originStoppable->id,
    'destination_id' => $destinationStoppable->id,
]);

1. To retrieve the origin and destination for a shipment, you can simply use the origin and destination relationships.

$shipment = Shipment::with(['origin.stoppable', 'destination.stoppable'])->find(1);

$origin = $shipment->origin->stoppable; // This will be an instance of CompanyLocation or Carrier
$destination = $shipment->destination->stoppable; // This will be an instance of CompanyLocation or Carrier

This setup allows you to have a flexible system where a shipment's origin and destination can be either a company location or a carrier, and you can easily extend it to include other types of stoppables in the future.