Shopping Cart Price Rule

Magento – Create a custom shopping cart price rule

Mar 8, 2012 Magento observer shopping cart price rule

This is not a tutorial about setting up a Shopping Cart Price Rule in Magento, but rather about implementing a new one.

A new type of rule in Magento needs a couple of things:
– modify the admin area to add the new rule using an observer for adminhtml_block_salesrule_actions_prepareform,
– a way to apply the new rule using an observer for salesrule_validator_process.

Let’s build an example. Let’s say there is a Shopping Cart Price Rule that offers different discounts according to the number of products in the cart. The value that’s going to be used for the discount increment ($step) will be calculated. The first product will not receive a discount, the second product will receive a discount of $step, the third product will have a discount of 2*$step, until the maximum discount value will be reached. The following products will have a maximum discount. Ex:
Discount Amount = 50
Discount Qty = 5
Step = Discount Amount / Discount Qty = 10

Discount outcome:
– 0% prod 1
– 10% prod 2
…
– 50% prod 6
– 50% prod 7

The first step is the module activation using the file: app/etc/modules/CP_ProductNrDiscount.xml

1<?xml version="1.0" encoding="UTF-8"?>
2<config>
3    <modules>
4        <CP_ProductNrDiscount>
5            <active>true</active>
6            <codePool>local</codePool>
7        </CP_ProductNrDiscount>
8    </modules>
9</config>

The first observer, adminhtml_block_salesrule_actions_prepareform, must be in the “adminhtml” section of the config, because it will involve the admin. This observer will have access to the admin form, in order to modify it.

The second observer, salesrule_validator_process, can be in the “frontend” or “global” section of the config. If it’s in the frontend section, it will only apply to the frontend section. If it’s in the global section it will also apply to backend. Usually, global is necessary when there are actions on the cart in the backend.

 1<?xml version="1.0" encoding="UTF-8"?>
 2<config>
 3    <modules>
 4        <CP_ProductNrDiscount>
 5            <version>0.0.1</version>
 6        </CP_ProductNrDiscount>
 7    </modules>
 8    <global>
 9        <models>
10            <productnrdiscount>
11                <class>CP_ProductNrDiscount_Model</class>
12            </productnrdiscount>
13        </models>
14        <events>
15            <salesrule_validator_process>
16                <observers>
17                    <productnrdiscount>
18                        <type>model</type>
19                        <class>productnrdiscount/observer</class>
20                        <method>salesruleValidatorProcess</method>
21                    </productnrdiscount>
22                </observers>
23            </salesrule_validator_process>
24        </events>
25    </global>
26    <adminhtml>
27        <events>
28            <adminhtml_block_salesrule_actions_prepareform>
29            <observers>
30                <productnrdiscount>
31                    <type>model</type>
32                    <class>productnrdiscount/observer</class>
33                <method>adminhtmlBlockSalesruleActionsPrepareform</method>
34                </productnrdiscount>
35            </observers>
36            </adminhtml_block_salesrule_actions_prepareform>
37        </events>
38    </adminhtml>
39</config>

As you can see above, there must be an Observer model that will have the two methods which modify the admin and apply the discount.

 1<?php
 2/**
 3 * Number of product discount module
 4 *
 5 * @author Claudiu Persoiu https://blog.claudiupersoiu.ro
 6 */
 7class CP_ProductNrDiscount_Model_Observer {
 8
 9    // The new rule type
10    const PRODUCT_NR_DISCOUNT = 'product_nr_discount';
11
12    /**
13     * Add the new rule type to the admin menu
14     *
15     * @param Varien_Event_Observer $observer
16     */
17    public function adminhtmlBlockSalesruleActionsPrepareform
18              (Varien_Event_Observer $observer) {
19        // Extract the form field
20        $field = $observer->getForm()->getElement('simple_action');
21        // Extract the field values
22        $options = $field->getValues();
23        // Add the new value
24        $options[] = array(
25            'value' => self::PRODUCT_NR_DISCOUNT,
26            'label' => 'Product Number Discount'
27        );
28        // Set the field
29        $field->setValues($options);
30    }
31
32    /**
33     * Apply the discount
34     * The discount will be applied for at least 2 products increasing
35     * with a "step" for each product, where "step" is
36     * maximum discount / number of products.
37     *
38     * @param Varien_Event_Observer $observer
39     */
40    public function salesruleValidatorProcess(Varien_Event_Observer $observer) {
41
42        // $item typeof Mage_Sales_Model_Quote_Item
43        $item = $observer->getEvent()->getItem();
44        // $rule typeof Mage_SalesRule_Model_Rule
45        $rule = $observer->getEvent()->getRule();
46
47        // Number of products
48        $qty = $item->getQty();
49
50        // We must check the rule type in order to isolate our rule type
51        if($rule->getSimpleAction() == self::PRODUCT_NR_DISCOUNT && $qty > 1) {
52
53            // Extract rule details
54            $discountAmount = $rule->getDiscountAmount();
55            $discountQty = $rule->getDiscountQty();
56
57            // Discount step
58            $step = $discountAmount/$discountQty;
59
60            // Discount calculation
61            $discount = 0;
62            for($i = 1; $i < $qty; $i++) {
63                $itemDiscount = $i * $step;
64                // If the discount is bigger then the maximum discount
65                // then the maximum discount is used
66                if($itemDiscount > $discountAmount) {
67                    $itemDiscount = $discountAmount;
68                }
69
70                $discount += $itemDiscount;
71            }
72            // Effective discount
73            $totalDiscountAmount = ($item->getPrice() * $discount)/100;
74
75            // Discount in percent for each item
76            $item->setDiscountPercent($discount / $qty);
77
78            // Setting up the effective discount, basically this is the discount value
79            $result = $observer->getResult();
80            $result->setDiscountAmount($totalDiscountAmount);
81            $result->setBaseDiscountAmount($totalDiscountAmount);
82
83        }
84    }
85
86}

This observer will run at each request if there are items in cart that for which the rule is applicable. If the discount should be applied only for specific products, there can be filtered using the rule’s “Conditions” tab, just as you would normally do.