public function whereEmpty($field)
    {
        return $this->where(
            '$or',
            array(
                array($field => null),
                array($field => ''),
                array($field => array()),
                array($field => array('$exists' => false))
            )
        );
    }
public function whereEmpty($field)
    {
        return $this->where(
            '$or',
            array(
                array($field => null),
                array($field => ''),
                array($field => array()),
                array($field => array('$exists' => false))
            )
        );
    }
public function whereEmpty ( $ field ) { return $ this -> where ( '$or' , array ( array ( $ field => null ) , array ( $ field => '' ) , array ( $ field => array ( ) ) , array ( $ field => array ( '$exists' => false ) ) ) ) ; }
Filter empty field

@param string $field

@return Expression
Filter empty field