Using a textfield to change form hidden field

I'm using Jquery with Ruby on Rails (3.1) and am having some problem with updating a hidden field via javascript.

Basically I have a textfield, and I'm trying to use ajax to update two different form hidden fields (one form for google checkout, one for amazon payments).

My Javascript looks like so:

$(function() {
  $('#payment_amount input').keyup(function() {
    var quantity = $('#payment_amount input').val();
    $("input[name='amount']").val("USD" + quantity);
    $("input[name='item_price_1']").val(quantity);
    return false;
  });
});

I have this in my application.js file for the time being (not yet in my .js.coffee, need to look into coffee script yet).

I think this javascript is correct, as I take the value from the input field, and it should update the input fields with the names 'amount' and 'item_price_1'. I thought maybe my selectors were wrong, so here's my html:

<input id="payment_amount" type="text" size="30" name="payment[amount]"> #textfield to change form hidden fiels

<form method="POST" action="https://checkout.google.com/api/checkout/v2/checkoutForm/Merchant/.... accept-charset="utf-8">
  <input type="hidden" name="item_price_1" value="20"/开发者_如何学Python> #what I want to change
  ....
</form>

<form action="https://authorize.payments.amazon.com/pba/paypipeline" method="post">
<input type="hidden" name="amount" value="USD 20" > #also change this
</form>

This is all comes from a new.html.erb file. I do not have a new.js.erb file, do I need one?

Right now, this javascript doesn't seem to do a thing.

Any help would be appreciated.

Change this line:

$('#payment_amount input').keyup(function() {
     var quantity = $('#payment_amount input').val();

To this:

$('#payment_amount').keyup(function() {
     var quantity = $(this).val();

You're using the wrong selector for your payment_amount field. You can see it working here.