Ruby on Rails - update fields based on f.select value - ruby

Question

I have a dropdown box to chose the number of tickets that one wants to buy. I want to update the fields after it to reflect the value once it is clicked. Here is a snippet of my form currently:
<%= form_for :transaction, :url => new_transaction_path(:event_id => #event.id), :method => 'GET' do |f| %>
<table style="width:100%">
<tr style="border-bottom: 1px solid #999999">
<td><h4>Number of Guests</h4></td>
<td>
<%= f.select(:quantity, (1..20))%>
</td>
</tr>
<tr style="border-bottom: 1px solid #999999">
<!-- replace (1) with the value from the f.select dropdown -->
<td><h4><%= #original_event_price %> x (1) guest(s)</h4></td>
</tr>
</table>
<%= f.submit 'Request to Book', class: 'button mt1 btn-request-to-book' %>
<% end %>
I want to replace (1) with the value from the f.select dropdown in the last table row to the value of guests that the user chooses.
EDIT WITH WORKING SOLUTION
<%= form_for :transaction, :url => new_transaction_path(:event_id => #event.id), :method => 'GET' do |f| %>
<table style="width:100%">
<tr style="border-bottom: 1px solid #999999">
<td><h4>Number of Guests</h4></td>
<td>
<%= f.select :quantity, (1..20), {}, { :onChange=>'mytest()', :id=>'quantitySelect' } %>
</td>
</tr>
<tr style="border-bottom: 1px solid #999999">
<!-- replace (1) with the value from the f.select dropdown -->
<td><h4><%= #original_event_price %> x (1) guest(s)</h4></td>
</tr>
</table>
<%= f.submit 'Request to Book', class: 'button mt1 btn-request-to-book' %>
<% end %>
SCRIPT
<script type="text/javascript">
function mytest() {
var quantity = $('#quantitySelect').val();
$('#quantityRow').html("<h4><%= #original_event_price %> x (" + quantity + ") guest(s)");
}
</script>

Answer 1

Problem : How to load value in other field based on drop-down selection
Solution:
<div class="field form-group">
<%= form.label :grade %>
<%= form.select(:grade, options_for_select(['A', 'B', 'C', 'D', 'E', 'F']), {:include_blank => 'Select Grade'}, class:"form-control", placeholder:"Grade", onchange: "loadRemarksBasedOnGrade()") %>
</div>
<div class="field form-group">
<%= form.label :remarks %>
<%= form.text_field :remarks, class:"form-control", placeholder:"Remarks", readonly: true %>
</div>
JavaScript :
<script type="text/javascript">
function loadRemarksBasedOnGrade() {
var grades = [];
grades['A'] = "Excellent";
grades['B'] = "Very Good";
grades['C'] = "Good";
grades['D'] = "Average";
grades['E'] = "Below average";
grades['F'] = "Failed";
let grade = document.getElementById("student_grade");
if(grades[grade.value] !== undefined) {
document.getElementById("student_remarks").value = grades[grade.value];
} else {
document.getElementById("student_remarks").value = "";
}
}
</script>
Note: So onchnage event loadRemarksBasedOnGrade update value based on id.

Answer 2

You can do this with jQuery:
Let's add an id: "quantitySelect" on the quantity field
Let's also add an id: "quantityRow" on the td field
$(function(){
$('.quantitySelect').change(function(e){
e.preventDefault();
var quantity = $('.quantitySelect').val();
$('.quantityRow').html("<h4><%= #original_event_price %> x (" + quantity + ") guest(s)");
});
});
You can also add if checks to see if the quantity is > 1 and append the word guest to guests.