Doctrine\MongoDB\Query\Expr::operator PHP Method

    public function operator($operator, $value)
    {
        $this->wrapEqualityCriteria();
        if ($this->currentField) {
            $this->query[$this->currentField][$operator] = $value;
        } else {
            $this->query[$operator] = $value;
        }
        return $this;
    }