protected function intermediateColumns()
    {
        $columns = [];

        foreach ($this->intermediateTables as $accessor => $intermediateTable) {
            $prefix = $this->prefix($accessor);

            foreach ($intermediateTable['columns'] as $column) {
                $columns[] = $intermediateTable['table'].'.'.$column.' as '.$prefix.$column;
            }
        }

        return array_unique($columns);
    }