I had the exact same problem and wasn’t very used to javascript.

Thanks for this post.
After two days of trying, this made my day.

However my solution is a little different.

In the check_box_tag I passed the checkbox to the function in the onchange handler like this.

:onchange => doChange(this)

and reffered to the select_tag in javascript like this.

function doChange(element){
element.form.select_player.disabled = element.checked
}