Call to undefined method Illuminate\Database\Eloquent\Builder::addEagerConstraints()

@theone you might need to put get() on your method:

public function getServices()
{
	return Services::whereIn('id',explode(',',$this->extra_service))->get();
}

Also, try calling it like this:

Booking::with('getServices')->get();

@theone you'll not be able to use it this way

Relation can't depends on current model value (like you try to use it here whereIn('id',explode(',',$this->extra_service)), because this value is different for every specific model, and you can't get ot for collection at once

I recommend to use pivot table to store selected services. This way you'll be able to use eager loading

Try using scopes, it will solve your problems https://laravel.com/docs/8.x/eloquent#query-scopes