# How the Cart Validation Works in the Frontend

#### 1) Basic Requirements

* The product must be configured as a rental item (tab Specifications → Rental).
* All relevant plugin settings (e.g. net/gross calculation, calendar, opening hours) must be saved.
* Afterwards, clear the cache if necessary.

***

#### 2) When clicking “Add to cart,” the plugin performs the following checks — in this order:

<br>

**A. Date Overlap (always active, regardless of parallel rentals)**

* The plugin checks whether the selected rental period overlaps with any existing reservation.
* If yes, and parallel rentals are not allowed (or Max. Parallel = 0/empty),

  the item cannot be added to the cart.
* The frontend displays an error message (snippet e.g. notAvailable).

***

**B. Parallel Rentals (only if enabled)**

* If “Check parallel rentals” is enabled for the product, the plugin determines:
  * the already reserved quantity during the requested period (from the reservation table),
  * the quantity already in the current cart,
  * and the currently requested quantity.
* These values are added up to form the total planned quantity.
* If the total planned quantity exceeds the product’s setting for “Max. parallel rentals,”

  the addition to the cart is rejected.
* The error message shows the allowed maximum (snippet e.g. maxParallelDays).

***

**C. Quantity Limit per Add-to-Cart**

* If the customer increases the quantity (e.g. “30 items”), this quantity is also included in the total planned quantity.
* Even if there are no items yet in the cart, the system blocks the addition if

  requested quantity + existing reservations > Max. Parallel.

***

#### 3) Examples

* Parallel rentals disabled

  An existing reservation: Oct 22 – Oct 28

  Requested period: Oct 21 – Oct 29 → Overlaps → Rejected (regardless of quantity).
* Parallel rentals = 2

  One reservation already exists.

  Customer adds quantity 2 → total = 1 + 2 = 3 > 2 → Rejected.

  Customer adds quantity 1 → total = 1 + 1 = 2 → Allowed.

***

#### 4) Display and Messages

* All error and info texts are managed via snippets (e.g. notAvailable, maxParallelDays, minDays, maxDays).
* This means all messages are fully translatable.

***

#### 5) Technical Overview (no code)

* The logic runs before the actual add-to-cart process.
* It considers both:
  * existing reservations in the requested period, and
  * items already in the current cart.
* Only if all checks pass will the rental item be added successfully.

***

> 💡 Tester Tip:

> Ensure the product is correctly configured as a rental item (prices, min/max days, parallel settings, etc.)

> and clear the shop cache before testing — this ensures all configuration and snippet changes are applied properly.


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