Laravel 4 Relation table column orderBy scope

RelationOrderScopeTrait.php

<?php namespace Site\Traits;

use Illuminate\Database\Eloquent\Relations\BelongsTo;
use Illuminate\Database\Eloquent\Relations\HasOne;
use Illuminate\Database\Eloquent\Builder;

trait RelationOrderScopeTrait {

    /**
     * Order also and by related table column
     *
     * $orderByData = ['relation.columnName' , 'asc'];
     *
     * @param  Builder  $builder
     * @param  array    $orderByArray
     * @return Builder
     */
    public function scopeSmartOrderBy($builder, array $orderByArray)
    {
        $model = $builder->getModel();

        list($orderColumn, $orderDirection) = $orderByArray;

        $this->assignArrayByPath($orderByData, $orderColumn, $orderDirection);

        foreach ($orderByData as $sort => $order)
        {
            if (is_array($order))
            {
                foreach ($order as $column => $value)
                {
                    $relation = $model->{$sort}();

                    if ($relation instanceof BelongsTo or $relation instanceof HasOne)
                    {
                        return $builder->join(
                                $relation->getRelated()->getTable().' as '.$sort,
                                $sort.'.'.$relation->getOtherKey(),
                                '=',
                                $relation->getForeignKey()
                            )
                            ->orderBy($sort.'.'.$column, $value)
                            // select model.* to be able to retrieve properties well
                            ->select($model->getTable().'.*');
                    }
                }
            }

            return $builder->orderBy($model->getTable().'.'.$sort, $order);
        }

        return $builder;
    }

    /**
     * @param  array   $arr
     * @param  string  $path
     * @param  mixed   $value
     *
     * @return void
     */
    private function assignArrayByPath(&$arr, $path, $value)
    {
        $keys = explode('.', $path);

        while ($key = array_shift($keys)) {
            $arr = &$arr[$key];
        }

        $arr = $value;
    }
}

Laravel 4.2
Try to use this, but got "method does not exist":

$relation->getOtherKey()