When box is checked make a drop down readonly or uneditable in Rails - ruby-on-rails

Question

I have a drop down box in ruby on rails that allows a date to be entered. And there is a box beside it to indicate that no date will be entered.
When this box is ticked it changes the default value of the date to nil. However the user can still enter a date on the drop down boxes.
When the box is ticked I want the drop down to be readonly or uneditable how can I do this?
EDIT
function BlankOutStartDates() {
if (document.getElementById("service_unknown_start_date").checked == true)
{
document.getElementById("service_start_date_1i").selectedIndex = 0;
document.getElementById("service_start_date_2i").selectedIndex = 0;
document.getElementById("service_start_date_3i").selectedIndex = 0;
}
}
I want to add the addition javascript to make the drop down box read only if the box is ticket

Answer 1

I would actually do this in jQuery. Suppose your checkbox has class "no-date", and your select boxes all of the class "date-select". Then you could do this:
jQuery(".no-date").change(function(){
//Check if the box is checked
if(this.is(':checked')){
jQuery(".date-select").attr("disabled", "true");
} else {
jQuery(".date-select").removeAttr("disabled");
}
});
You'll want to double check "this.is(':checked')" is correct, but I think that will work.
This uses the "disabled" parameter of select tags: http://www.w3schools.com/tags/tag_select.asp