What you need to know about debugging Magento cart price rules

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Sometimes you have the problem that a Magento cart price rule does not work when you enter a coupon code, although you think it should.

There are certain areas where it could fail and I will illustrate the two most important ones.

First check for the file Mage_SalesRule_Model_Validator and its protected function _canProcessRule($rule, $address).
The rule fails inside that function and this can be for many reasons which I would divide into two main categories:

a) General limitations such as a usage limit (code can only be used 5 times), customer limit (can only be used once per customer)
b) If the general limitations do not apply, the rule will fail at if (!$rule->validate($address)) so at its validate() method.
To debug this, I found that it is most useful to go to the file Mage_Rule_Model_Condition_Combine and in the function validate() enter the following code:

        foreach ($this->getConditions() as $cond) {
            $validated = $cond->validate($object);
            // MP These lines added
            $session = Mage::getSingleton('checkout/session');
            if ($validated !== $true) {
               $session->addError($cond->asString());
            }
Now when you enter the code and the rule fails, you will see a nice error in your cart which condition failed.

Hope this helps!