如何使用MongoDB的位置运算符$ []生成不同的ObjectId？

Question

我想使用MongoDB位置运算符days更新数组$[]中的每个元素（我有7天，这里只显示3天）。更新正确地将一个对象推入数组meals的每个元素中。但是，它们的对象ID相同，并且我希望它们有所不同，因为我将对其进行映射。有什么想法吗？

    "_id": {
        "$oid": "5db749cf657e1b1e0c3d241c"
    },
    "finished": false,
    "days": [
        {
            "dayName": "Monday",
            "meals": [
                {
                    "_id": {
                        "$oid": "5dba1cde12f2b650e4b4f89d"
                    },
                    "mealName": "Breakfast",
                    "recipes": []
                }
            ]
        },
        {
            "dayName": "Tuesday",
            "meals": [
                {
                    "_id": {
                        "$oid": "5dba1cde12f2b650e4b4f89d"
                    },
                    "mealName": "Breakfast",
                    "recipes": []
                }
            ]
        },
        {
            "dayName": "Wednesday",
            "meals": [
                {
                    "_id": {
                        "$oid": "5dba1cde12f2b650e4b4f89d"
                    },
                    "mealName": "Breakfast",
                    "recipes": []
                }
            ],
        }
    ]
}