Craft Commerce: How do I handle quantity based discounts on a product level?

Question

I'm curious what the best way to handle quantity discounts would be on a product/variant level. For example:

The base price of a variant is $3 each, but if you buy 10-19 they're $2 each, and if you buy more than 20 they're $1 each.

A different variant of the same product could have a base price of $10 each, but if you buy 20-49 they're $8 each, and if you buy over 50 they're $6 each.

Thoughts?

EDIT 2 (added file paths)

Based on the comments by Luke and Andris below, I've implemented an adjuster to handle this. I created a table field with the handle quantityPricing with 3 columns: minQuantity, maxQuantity, price. I then applied this field to the variants on a product.

This way a user can create as many prices for quantity ranges as they want. I then create the adjuster, checking the line items via the following:

craft/plugins/myplugin/Adjusters/Myplugin_Adjustername.php

<?php

namespace Commerce\Adjusters;
use Craft\Commerce_LineItemModel;
use Craft\Commerce_OrderAdjustmentModel;
use Craft\Commerce_OrderModel;

class Myplugin_Adjustername implements Commerce_AdjusterInterface {
public function adjust(Commerce_OrderModel &$order, array $lineItems = [])
{

    if (empty($lineItems))
    {
        return [];
    }

    $updated  = [];
    $amount   = 0;
    $adjuster = new Commerce_OrderAdjustmentModel();

    $adjuster->type        = 'Discount';
    $adjuster->name        = 'Quantity Pricing';
    $adjuster->description = 'The overall price of the cart has been adjsuted based on the number of items you are purchasing.';
    $adjuster->orderId     = $order->id;

    foreach ($lineItems as $lineItem)
    {
        $purchasable = $lineItem->purchasable;
        if (!empty($purchasable->quantityPricing))
        {
            $updated[]     = $lineItem->id;
            $updatedAmount = 0;
            foreach ($purchasable->quantityPricing as $qp)
            {
                if (
                    ($lineItem->qty >= $qp['minQuantity'] && $lineItem->qty <= $qp['maxQuantity'])
                 ||
                    ($lineItem->qty >= $qp['minQuantity'] && empty($qp['maxQuantity']))
                )
                {
                    $lineItem->discount = ($qp['price'] - $lineItem->price) * $lineItem->qty;
                    $updatedAmount      = $lineItem->discount;
                }
            }
            $amount += $updatedAmount;
        }
    }

    $adjuster->amount      = $amount;
    $adjuster->optionsJson = ['lineItemsAffected' => $updated];

    $adjuster->included = false;

    if (empty($updated))
    {
        return[];
    }

    return [$adjuster];
}
}

I was also seemingly able to do this using onPopulateLineItem but based on the advice below changed it. My code for that looked like this:

    craft()->on('commerce_lineItems.onPopulateLineItem', function($event){

            $purchasable = $event->params['purchasable'];
            $lineItem    = $event->params['lineItem'];

            // This method checks for quantity pricing and updates 
            // line items costs depending on what it finds.
            if (!empty($purchasable->quantityPricing))
            {
                foreach ($purchasable->quantityPricing as $qp)
                {
                    if (
                        ($lineItem->qty >= $qp['minQuantity'] && $lineItem->qty <= $qp['maxQuantity'])
                     ||
                        ($lineItem->qty >= $qp['minQuantity'] && empty($qp['maxQuantity']))
                    )
                    {
                        $lineItem->price = (float) $qp['price'];
                    }
                }
            }
        });

As mentioned in the comments below, I'm not exactly sure why an Adjuster is the better route to go, because both seem to work.

Please don't hesitate to mention if I did something ridiculously stupid somewhere. It certainly wouldn't be the first time.

Answer 1

Have you looked into building an adjuster for this purpose? You can find the documentation here