Magento – Magento coupon codes – only one coupon / guest customer

Here's an approach that I took to put in a better error message for two specific filters: the from date and to date.

Those filters are pretty simple compared to digging into the actual rule conditions such as subtotal, as you've mentioned in your question, but I think still provide a significant usability improvement with a pretty straight forward implementation.

There are two pretty clean rewrites that can be done to accomplish this.

Mage_SalesRule_Model_Resource_Rule_Collection::addWebsiteGroupDateFilter

Overload the addWebsiteGroupDateFilter method to prevent rules that don't match the date filter from being excluded entirely from the rules that are processed.

public function addWebsiteGroupDateFilter($websiteId, $customerGroupId, $now = null)
{
    parent::addWebsiteGroupDateFilter($websiteId, $customerGroupId, $now);

    $where = $this->_removeDateFilters();
    $this->getSelect()->setPart('where', $where);

    return $this;
}

protected function _removeDateFilters()
{
    $where = $this->getSelect()->getPart('where');

    foreach ($where as $index => $whereLine) {
        if (strpos($whereLine, "from_date is null or from_date <") !== false) {
            unset($where[$index]);
        } elseif (strpos($whereLine, "to_date is null or to_date >") !== false) {
            unset($where[$index]);
        }
    }

    $where = array_values($where);

    return $where;
}

Clean_Checkout_Model_SalesRule_Validator::_canProcessRule

Overload the _canProcessRule method to check the dates and add a specific error message to the session.

protected function _canProcessRule($rule, $address)
{
    if ($this->_isRuleExpired($rule, $address)) {
        return false;
    }

    return parent::_canProcessRule($rule, $address);
}

protected function _isRuleExpired($rule, $address)
{
    if ($rule->getFromDate() && date('Y-m-d', time()) < $rule->getFromDate()) {
        $message = "This coupon won't be active until {$rule->getFromDate()}";
        Mage::getSingleton('checkout/session')->addUniqueMessages(new Mage_Core_Model_Message_Error($message));

        return true;
    }

    if ($rule->getToDate() && date('Y-m-d', time()) > $rule->getToDate()) {
        $message = "This coupon expired on {$rule->getToDate()}";
        Mage::getSingleton('checkout/session')->addUniqueMessages(new Mage_Core_Model_Message_Error($message));

        return true;
    }
}

You can do this simply with some javascript. The javascript implementation can be done in a few ways. This is but one.

The quickest would be to copy the core template to your theme

/app/design/frontend/base/default/template/checkout/cart/coupon.phtml

and then set the coupon element to display:none with some in-line css (or you can adjust the carts stylesheet if you do not want to use in-line css, which is better)

Then all you have to do is place your 'click here' element to have a click event to toggle the coupon display and hide the click element.

Have not tested this code for 100% correctiveness

<form id="discount-coupon-form" action="<?php echo $this->getUrl('checkout/cart/couponPost') ?>" method="post">
    <a class="show-coupon-box" href="#">Have a coupon? Click to enter</a>
    <div class="discount" style="display:none">
        <h2><?php echo $this->__('Discount Codes') ?></h2>
        <div class="discount-form">
            <label for="coupon_code"><?php echo $this->__('Enter your coupon code if you have one.') ?></label>
            <input type="hidden" name="remove" id="remove-coupone" value="0" />
            <div class="input-box">
                <input class="input-text" id="coupon_code" name="coupon_code" value="<?php echo $this->escapeHtml($this->getCouponCode()) ?>" />
            </div>
            <div class="buttons-set">
                <button type="button" title="<?php echo $this->__('Apply Coupon') ?>" class="button" onclick="discountForm.submit(false)" value="<?php echo $this->__('Apply Coupon') ?>"><span><span><?php echo $this->__('Apply Coupon') ?></span></span></button>
                <?php if(strlen($this->getCouponCode())): ?>
                    &nbsp; <button type="button" title="<?php echo $this->__('Cancel Coupon') ?>" class="button" onclick="discountForm.submit(true)" value="<?php echo $this->__('Cancel Coupon') ?>"><span><span><?php echo $this->__('Cancel Coupon') ?></span></span></button>
                <?php endif;?>
            </div>
        </div>
    </div>
</form>

and some javascript to deal with the click

$$('.show-coupon-box')[0].observe('click', function(e){
   $$('.discount')[0].toggle(); $$('.show-coupon-box')[0].toggle(); 
   e.stop();
});