assets/javascripts/assignee_type.js
$(document).ready(function(){
var showAssigneeSelectOnlyForUser = function() {
$('select.hook_assigned_to_id').parent().toggle(
$('select.hook_assignee_type').val() == 'user'
);
};
$('select.hook_assignee_type').on('change', showAssigneeSelectOnlyForUser);
showAssigneeSelectOnlyForUser();
});