code: public function roots() {
      return $this->node->newQuery()
        ->whereNull($this->node->getQualifiedParentColumnName())
        ->orderBy($this->node->getQualifiedLeftColumnName())
        ->orderBy($this->node->getQualifiedRightColumnName())
        ->orderBy($this->node->getQualifiedKeyName())
        ->get();
    }
docs: Return all root nodes for the current database table appropiately sorted.
  
  @return Illuminate\Database\Eloquent\Collection