grails: sum selected values - grails

Question

I have a select in Grails as follows
<g:select name="receiptItems" from="${myGrailsProject.ReceiptItem.list()}" multiple="multiple" optionKey="id" optionValue="description" size="5" value="${receiptInstance?.receiptItems*.id}" class="many-to-many"/>
ReceiptItem object has a numeric field (amount). I need to do the sum of amount fields of selected values and put it in another textfield.
In particular, when I select a new value I need to add it to the "totalAmount", and viceversa, when I deselect a value, I need to remove that value from the "totalAmount".
How can I understand if the item is selected or unselected?How can I perform the calculation into the Controller class and then update the textfield with new value in javascript?
Thanks for your help

Answer 1

You can use jquery to do that. For example, your select is
<select id="mySelect" multiple="multiple">
<option value="1">First</option>
<option value="2">Second</option>
<option value="3">Third</option>
<option value="4">Fourth</option>
</select>
listen for the change event:
$('#mySelect').change(function() {
var amount = 0;
$('#mySelect option').each(function() {
if($(this).is(':selected'))
amount += $(this).val();
});
$('#amount').val(amount);
});
Hope this helps.

Answer 2

Ok so you have a list of IDs of objects and you need to sum a field of that domain and return the sum to javascript? If so:
JavaScript:
$('#mySelect').change(function() {
var $this = $(this);
$.ajax({
type: 'POST',
url: enterYourPathHere,
data: {values: $this.val()},
dataType: 'json',
success: function(data) {
//Populate your totalAmount field with data.amount
},
error: function(jqXHR, textStatus, errorThrown) {
//Handle any errors
}
});
Controller:
Float totalAmount = Object.findByIdInList(params.list("values"))?.amount?.sum() ?: 0
render [amount: totalAmount] as JSON
Obviously change this for your Domain, fields, etc...