I had to do this exact same thing on my web app just the other day, so I thought I’d share with you what worked for me. In my case, I wanted to be able to search my user records for users that were in the cities I selected. The key is knowing that a multiple selection drop down list submits the result as an array.
So, first, I changed the drop down list in my search form to a multiple select list…
Then, you just need to edit the search() method in your model and change the attribute you’re searching on to an IN condition (rather than a ‘compare’ condition)…