League\Monga\Query\Update::addToSet PHP Method

Adds the values to the set or upsert when the dataset doesn't contain them already

public addToSet ( string $field, mixed $value ) : object
$field	string	field name
$value	mixed	value to set/upsert with
return	object	$this

    public function addToSet($field, $value)
    {
        return $this->update('$addToSet', $field, $value);
    }

Update