Using CodeIgniter to get the Last Inserted ID

I often need to know the id of the last inserted record into a table. In the past, I’ve used a PHP function:

function getLastInserted($table, $id) {
	$query = "SELECT $id as maxID from $table where $id = LAST_INSERT_ID()";
	$result = $this->runQuery($query);
	$row = mysql_fetch_row($result);
	return $row[0];

To duplicate this using CodeIgniter’s Active Record, I could write:

function getLastInserted($table, $id) {
	$Q = $this->db->get($table);
	$row = $Q->row_array();
	return $row[$id];

But, I learned that there is an even faster way that uses one of CodeIgniter’s Helper functions. Immediately after a record has been inserted into the database, I simply call:

$id = $this->db->insert_id();

Although, I am not sure how CodeIgniter knows which field is the primary key, it seems to work.

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