When I add a specific ticket type of an event to the cart, the simplest way is to call the add to cart API [POST] and add a custom property to in an event item in the cart.

Yet, this is not a workable solution because:

• Value of the custom property will not be accelerated
• Value of the custom property will be overwritten if I send the request with data that has the same product id

For example, here is a data of an event, "夏日序曲 — 莫扎特的交響樂" :

{
        "category": "音樂",
        "dateTime": [
            {
                "date": "2021-08-11",
                "endTime": "13:30",
                "startTime": "12:00",
                "timestamp": 1625891831522
            },
            ...
        ],
        "id": "-MeDhhevaeV5e8wUDUuS",
        "ticketPrice": {
            "A": 240,
            "B": 360,
            "C": 420
        },
        "title": "夏日序曲 — 莫扎特的交響樂",
        ...
},

Here is the data of the cart after a user adds a specific ticket type to the cart:

"carts": [
    {
        "1625891831522,A區,正價票": 1,
        "id": "-Mn-feaX5aMbyXZUrHrs",
        "product": {
            "title": "夏日序曲 — 莫扎特的交響樂",
            ...
        },
        "product_id": "-MeDhhevaeV5e8wUDUuS",
        ...
    }
]

If user adds one more 1625891831522,A區,正價票 ticket, the value of 1625891831522,A區,正價票 will still be 1 instead of 2, because it overwrites the previous data:

"carts": [
    {
        "1625891831522,A區,正價票": 1,
        ...
    }
]

Same problem occurs if the user adds a different ticket type.

For example, if the user adds 1625891831522,C區,優惠票, then 1625891831522,A區,正價票 will be overwritten. Here is the result:

"carts": [
    {
        "1625891831522,C區,優惠票": 1,
        "id": "-Mn-feaX5aMbyXZUrHrs",
        "product": {
            "title": "夏日序曲 — 莫扎特的交響樂",
            ...
        },
        "product_id": "-MeDhhevaeV5e8wUDUuS",
        ...
    }
]

Call edit cart API if I have to accumulate the quantity

Since I was using the API developed by the bootcamp, changing the APIs was not an option for me and I had to solve it in front-end. To avoid overwriting the current data when I had to accumulate the quantity, I would call edit cart PUT API. Below is my algorithm for solving this problem.

The back-end calculates the total amount with the quantity property, not the value of a custom property.

In my case, I stored quantity of each ticket type with custom key and value, such as {1625854073524,A區,正價票: 1}. Because I did not store it with the given quantity property, the total amount was not calculated correctly. Worse still, if the user applied coupon, the discount value was calculated based on the total amount, which would be incorrect.