/**
* Find a model by its primary key.
*
* @param mixed $id
* @param array $columns
* @return \LMongo\Eloquent\Model|null
*/
public function find($id, $columns = array())
{
if (!$id instanceof MongoID) {
$id = new MongoID($id);
}
$this->query->where($this->model->getKeyName(), $id);
return $this->first($columns);
}