### What
Add a new `once` allocation strategy to promotions that limits application to a maximum number of items across the entire cart, rather than per line item.
### Why
Merchants want to create promotions that apply to a limited number of items across the entire cart. For example:
- "Get $10 off, applied to one item only"
- "20% off up to 2 items in your cart"
Current allocation strategies:
- `each`: Applies to each line item independently (respects `max_quantity` per item)
- `across`: Distributes proportionally across all items
Neither supports limiting total applications across the entire cart.
### How
Add `once` to the `ApplicationMethodAllocation` enum.
Behavior:
- Applies promotion to maximum `max_quantity` items across entire cart
- Always prioritizes lowest-priced eligible items first
- Distributes sequentially across items until quota exhausted
- Requires `max_quantity` field to be set
### Example Usage
**Scenario 1: Fixed discount**
```javascript
{
type: "fixed",
allocation: "once",
value: 10, // $10 off
max_quantity: 2 // Apply to 2 items max across cart
}
Cart:
- Item A: 3 units @ $100/unit
- Item B: 5 units @ $50/unit (lowest price)
Result: $20 discount on Item B (2 units × $10)
```
**Scenario 2: Distribution across items**
```javascript
{
type: "fixed",
allocation: "once",
value: 5,
max_quantity: 4
}
Cart:
- Item A: 2 units @ $50/unit
- Item B: 3 units @ $60/unit
Result:
- Item A: $10 discount (2 units × $5)
- Item B: $10 discount (2 units × $5, remaining quota)
```
**Scenario 3: Percentage discount - single item**
```javascript
{
type: "percentage",
allocation: "once",
value: 20, // 20% off
max_quantity: 3 // Apply to 3 items max
}
Cart:
- Item A: 5 units @ $100/unit
- Item B: 4 units @ $50/unit (lowest price)
Result: $30 discount on Item B (3 units × $50 × 20% = $30)
```
**Scenario 4: Percentage discount - distributed across items**
```javascript
{
type: "percentage",
allocation: "once",
value: 15, // 15% off
max_quantity: 5
}
Cart:
- Item A: 2 units @ $40/unit (lowest price)
- Item B: 4 units @ $80/unit
Result:
- Item A: $12 discount (2 units × $40 × 15% = $12)
- Item B: $36 discount (3 units × $80 × 15% = $36, remaining quota)
Total: $48 discount
```
**Scenario 5: Percentage with max_quantity = 1**
```javascript
{
type: "percentage",
allocation: "once",
value: 25, // 25% off
max_quantity: 1 // Only one item
}
Cart:
- Item A: 3 units @ $60/unit
- Item B: 2 units @ $30/unit (lowest price)
Result: $7.50 discount on Item B (1 unit × $30 × 25%)
```
what:
Introduces 2 new features to promotion module:
1. Introduce max quantity limit to promotion application - This will limit the application of the promotion based on the quantity of the target products in the cart.
2. When applying buy get promotions, we will now apply buyget promotion until eligible items are exhausted or max quantity is reached.
```
- Buy 2 t-shirts, Get 1 sweater
- Max quantity -> 1
This means you can add two t-shirts, and get 1 sweaters for free. However, if you add four t-shirts, you only get one sweater for free.
```
```
- Buy 2 t-shirts, Get 1 sweater
- Max quantity -> 3
This means you can add six t-shirts, and get three sweaters for free. However, if you add eight t-shirts, you only get three sweaters for free
```
```
- Buy 4 t-shirts, Get 2 sweater
- Max quantity -> 1
This should throw on creation, as the max quantity should as a minimum be the same value as the target rule quantity
```
RESOLVES SUP-2357 / https://github.com/medusajs/medusa/issues/13265
