CakePHP 3 : Retrieving data from database using Ajax - ajax

Question

I am working on a CakePHP 3 project which is having a Apply Coupon form.
I want to apply the coupon using Ajax.
The view of coupon form is
<?= $this->Form->create(null, [
'url' => ['controller' => 'Coupons', 'action' => 'checkCoupon'],
'name' => 'checkCoupon'
]) ?>
<?= $this->Form->input('coupon_code', [
'type' => 'text',
'placeholder' => 'Apply Coupon Code',
'label' => false
]) ?>
<?= $this->Form->submit('Apply Coupon') ?>
and the checkCoupon action in CouponsController is
public function checkCoupon()
{
$this->request->onlyAllow('ajax'); // No direct access via browser URL
if ($this->request->is('post')) {
$couponCode = $this->request->data['coupon_code'];
$couponCheck = $this->Coupons->find('all', [
'conditions' => [
'coupon_code' => $couponCode
]
]);
if ($couponCheck->count() === 1) {
$coupon = $couponCheck->first();
$valid_till = $coupon->valid_till;
$dt = new Time($valid_till);
$date = $dt->format('Y-m-d');
if ($date >= date('Y-m-d')) {
echo 'Coupon is Valid. Discount of '.$coupon->value.'has been applied';
} else {
echo 'Coupon is Expired';
}
} else {
echo 'This is not a valid coupon code';
}
}
}
I want the $coupon->value and $coupon->id to be retrieved and added to the checkout link as
<?= $this->Html->link(__('Confirm Checkout'), ['controller' => 'ServiceRequests', 'action' => 'confirmCheckout', $service->id, $primaryAddressId, $serviceArea->id, $coupon->id], ['class' => 'btn btn-block btn-success']) ?>
The Apply Coupon form is in checkout action of RequestsController
Also the form is working well. I have checked it by removing the onlyAllow('ajax') line and printing values in check_coupon.ctp view.
How could I do it using Ajax ?
Edit 2 : checkout.ctp
<div class="form-info coupon">
<?= $this->Form->create(null, [
'url' => ['controller' => 'Coupons', 'action' => 'ajax_checkCoupon'],
'name' => 'checkCoupon',
'id' => 'checkCoupon'
]) ?>
<?= $this->Form->input('coupon_code', [
'type' => 'text',
'placeholder' => 'Apply Coupon Code',
'label' => false
]) ?>
<label class="hvr-sweep-to-right">
<?= $this->Form->submit('Apply Coupon', ['id' => 'applyCoupon']) ?>
</label>
<label id="couponUpdate"></label>
<label id="loading" style="display:none;">Loading...</label>
<?php
$data = $this->Html->script('#checkCoupon')->serializeForm(['isForm' => true, 'inline' => true]);
$this->Html->script('#checkCoupon')->event(
'submit',
$this->Html->script(
[
'controller' => 'Coupons',
'action' => 'ajax_checkCoupon'
],
[
'update' => '#couponUpdate',
'data' => $data,
'async' => true,
'dataExpression' => true,
'before' => "$('#loading').fadeIn();$('#applyCoupon').attr('disabled','disabled');",
'complete' => "$('#loading').fadeOut();$('#applyCoupon').removeAttr('disabled');"
]
)
);
?>
</div>
Error : Call to a member function serializeForm() on string on line 18 in checkout.ctp