Implement freezePromotions for basic order modification admin flow

[dlhck]

Migrated from #4106

Post sale modification of an order is often required by webshop admins. Currently vendure ships a modify order detail component, where various order details can be changed:

• Order lines
• Shipping methods
• Billing and shipping addresses
• Promotions
• Surcharges

These more often than not impact the order total. Easiest example is orderLine changes: more or less items on the order results in additional payment request or refunds. I think this is easy to understand, if a customer requests more items, then they can be expected to pay more as well.

When a customer's address is changed, shipping method rates may also recalculate, or even just querying third party shipping rates API might give back different rate depending time of the day during order modifications.
To avoid this, we have checkbox recalculate shipping, to bypass order total changes.

What I think vendure misses right now for basic order admin flow, is the option to bypass order promotion recalculation as well. We can even find the option freezePromotions among graphql options, but it is not used in the core code.

I really like vendure's promotion system. Supports a ton of business goals, but with custom code. One thing is certain, that if a change happens to the order, it will trigger the applyPriceAdjustments function. That function according to the docs applies taxes and promotions to the order.

I think implementing freezePromotions can be easily done, but it can get tangled up with business goals, promotion terms & conditions. I assume that's why it wasn't implemented in the end.

Situation 1.):

• Run a promotional campaign for a limited time, like 10% off on specific products
• Customers place orders, promotion is recorded on the order
• Shop owner requires 5 days on average to prepare and package an order
• Campaign is over, but customer emails, requesting a slight address change on the order: as the campaign is over, upon order modification and triggering applyPriceAdjustments, the previously added promotion is removed and even though there was just an apartment number change request, the order totals change

In situation 1.) I think it's clear to see that a checkbox to avoid promotion recalculation comes in handy.

Situation 2:

• Run a promotion for a limited time, which is buy 2 product, get 10% off
• Now the customer changes mind, wants to just buy 1 product
• This case the order totals should be recalculated, after all the promotion t&c said that 10% off should be applied to order with 2 products only
• But promotion is inactive, as the limited time is over

In situation 2.) it would be great to recalculate the promotion, but due to it being for a limited time, it's not possible for now. I don't say it's a problem, but I wanted to write a situation to highlight nuances of promotions set by shop owners.

On my vendure fork I have started the implementation, but I would like to invite others to share any ideas related to implementing the missing freezePromotions option to give a smoother default experience for shop owners.

kkerti@cc23616

[kkerti]

I am ready to submit a PR on this. But it would be great to hear from the vendure core team any concerns related to this.

[dlhck]

I agree that the current behavior can be surprising for shop admins, especially in scenarios like your Situation 1 where a simple address change shouldn't affect pricing. That's a valid pain point.

That said, I think the implementation is less straightforward than it might seem at first glance. Consider a variation of your Situation 2: a time-limited "buy 2, get 10% off" promotion was applied to an order. The customer now wants to increase the quantity to 3. With freezePromotions enabled, the discount would automatically extend to the new item as well, which may not be what the commerce operator intended, especially after the campaign has ended.

So a simple boolean toggle doesn't fully capture the complexity here. The decision of when to freeze and what to freeze depends heavily on business rules that vary between merchants.

I think a strategy pattern would be a better fit for this. Something like a FreezePromotionStrategy that merchants can implement to define their own logic for whether promotions should be recalculated during order modifications. The default could preserve the current behavior (always recalculate), making it fully opt-in, while giving merchants the flexibility to handle their specific scenarios.

Would love to hear your thoughts on this direction before moving forward with a PR.