I can think of two ways to go about this if you don't want to load the contents initially when the page loads. One way is to run a script after the DOM has loaded to change the options for the select tag and the other is to collect the options when you click on the drop-down on the select element. I might go for the first way because there wouldn't be latency when a user clicks on the select element--they wouldn't have to wait for the options to populate.
So you'd run a jQuery script on document ready that makes an AJAX call to a method in your controller, which then returns the collections you want, then you iterate through the select elements you want to change with JQuery scripts. It might look something like this.
# in view with the select options to be changed
$(document).ready(function() {
$.get(change_selects_path, function(response) {
$.each(response, function(args) {
// code for each select element to be changed
$('.class_of_select_element').html(<%= j options_from_collection_for_select(args) %>);
});
});
)};
# in controller
def change_selects
# make db calls and store in variables to feed to $.get request
end
Note that this not tested but should give you a good start towards a solution. For further info on the each loop, you can check out this documentation.