no mysql_insert_id() after $this->db->setQuery( "INSERT

[Carsten888 eng]

mysql_insert_id() returns 0 after a succesfull insert.

It worked fine when I defined the insert with:
$database->setQuery( "INSERT....
$database->query();
echo mysql_insert_id();
//returns the id

but when I made a var from $database in a class the insert works but the id is 0
$this->db->setQuery( "INSERT....
$this->db->query();
echo mysql_insert_id();
//returns 0

I tried these, but thats just plane wrong.
echo mysql_insert_id($database);
echo mysql_insert_id($this->db);
which gives error:
Warning: mysql_insert_id(): supplied argument is not a valid MySQL-Link resource in....

anyone?