Magento provides a sales rule engine which allows you to offer discount to your clients: free shipping, buy x products and get y products free, providing a discount from a voucher are samples usage of prices rules
Two important concepts exist in this model:
- Can we cluster sale rules: can rules be combined or if a rule is applied, are the others automatically excluded ?
- Sorting rules importance within the priority concept
How does Magento apply prices rules on cart ?
Each time you update your cart content or each time you display the cart page, Magento loads all the active sales rules for the current date, current website, current user group and test for-each rule if the conditions match the cart content. If yes, Magento applies specified discounts to items in cart.
If a rule has been flagged as “stop rule processing”, we stop iteration on the rule collection. If not, we continue and apply the others matching discount until we have tested all rules or we have applied a discount which stop iteration
The Magento sales rule priority
Priority is a numeric id defining the rule importance and how it’ll be tested: the less is the priority value, the more important is the rule
Priority is specified when the sales rule is defined from back-office
Priority is not a mandatory field: if you do not specify a value, the priority applied will be 0, and so the rule will be tested at first before all other sale rules
Technically, when the rules collection is loaded, it’ll be sorted by the priority value (sort_order field)
Priority is not unique: you can define several rules having the same priority
What happens if some Magento sale rules have the same priority?
There is no second filter to order rules: so if you have two rules which can be applied, this is the oldest one which will be used and applied firstly. Is it really what you were expecting? Perhaps not.
Furthermore, if you have many sale rules, you should ensure to know exactly each conditions criteria to understand exactly how are applied rules and in which order. Possibly very difficult in a long time or if you have many sale rules defined
How can you reduce the risk that some rules are not executed as you want?
Reviewing the rules to ensure that priority is unique is the only one solution to avoid risk that rules are not executed the way you are expecting them. But it could be difficult to maintain if you define later others sales rules.
If you want to reorder priority, you can define priority levels: the most important one have a priority defined with one digit, lesser priority with two digits, oldest one or more generic sale rules with a priority of three digits. In this case, you’ll ensure that you’ll know with an unique priority how they’ll be tested, and be able to add others rules easily
One other best practice is to defind strong application conditions criteria: the more generic they are, the more risk you have to apply a rule that should not be executed