database class problem

[eluser]emperius[/eluser]
I created an associative array

Code:

$addarray = array('field1' => $_POST['field1'], 'field2' => $_POST['field2']);

then I try to add this string to DB

Code:

$this->db->insert_string('table', $addarray)

this doesn't work at me

and when I try to see the query string I get empty string...

How to find out what is the problem?
And how can I see the query?