Eloquent custom "belongs to" relationship on multiple tables - laravel

Question

I have the following DB structure:
Table Vehicles: id, car_id, plane_id
Table Cars: id, model...
Table Planes: id, model...
When a new record added to the table Vehicles, if it is a Car, the car_id will be set, while the plane_id will be left empty, and vice-versa, I know it's a bad structure, but it is legacy and I can't change it.
So I want to define a relationship in the Vehicle model where it can retrieve the Car object or the Plane object according to which key is empty, the car_id or the plane_id. And btw, I've already defined two relationships that will retrieve the Car object and the Plane object separately.
public function carVehicle()
{
return $this->belongsTo(Car::class, 'car_id');
}
public function planeVehicle()
{
return $this->belongsTo(Plane::class, 'plane_id');
}

Answer 1

This would be best handled by a Polymorphic relationship (see https://laravel.com/docs/5.8/eloquent-relationships#polymorphic-relationship for details), but there's ways around this if your current model doesn't match the structure and you can't change it.
You could have a third method that adds both to a Collection and returns the first() one (since you say one of car_id or plane_id will always be null):
Vehicle.php:
public function getChildVehicleAttribute(){
return collect([$this->carVehicle, $this->planeVehicle])
->filter(function($record) {
return $record != null;
})->first();
}
Then, you'd access via the following query:
$vehicle = Vehicle::with(['carVehicle', 'planeVehicle'])->first()->child_vehicle;
// OR
$vehicles = Vehicle::with(['carVehicle', 'planeVehicle'])->get();
foreach($vehicles AS $vehicle){
$childVehicle = $vehicle->child_vehicle;
// dd($childVehicle, etc.)
}
The with() clause would eagerload both relationships so $this->carVehicle and $this->planeVehicle don't trigger additional database calls, and calling child_vehicle on any Vehicle instance would return either a Car or a Plane (or null if neither is defined)