Active Record: how do I combine two or_like with an AND between them?

[eluser]sparbier[/eluser]
Hi all,
I need to run a query that takes inputs from two arrays (from post data, don't know beforehand how many elements are given), and looks for the elements of these arrays in two separate fields in my database. So this is what I did:

Code:

if (isset($_POST['region'])) {
        foreach ($_POST['region'] as $r)  {
            // put the searched id between commas before searching for it in the string, in order not to e.g. find a 4 contained in 42 but only ,4,
            $searchstring = ','. $r .',';
            $this->db->or_like('regions', $searchstring);
        }
    }
    if (isset($_POST['instrument'])) {  
        foreach ($_POST['instrument'] as $i)  {
            $searchstring = ','. $i .',';
            $this->db->or_like('instruments', $searchstring);
        }
    }

The issue now is, that while I do want the LIKE statements within each of the two criteria be linked with OR, the two criteria must be linked with AND.

Is there any way to achieve this with Active Records?