$arrayElemAt(数组表达式)
适用对象: MongoDB vCore
$arrayElemAt
运算符用于返回指定数组索引处的元素。 当需要从文档的数组中提取特定元素时,此运算符非常有用。
{ "$arrayElemAt": [ "<array>", "<idx>" ] }
说明 | |
---|---|
<array> |
从中检索元素的数组参考。 |
<idx> |
要返回的元素的索引。 该索引从零开始。 负索引从数组末尾开始计数。 |
让我们了解 stores
数据集中的示例 json 的用法。
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
"location": {"lat": 60.1441, "lon": -141.5012},
"staff": {"totalStaff": {"fullTime": 2, "partTime": 0}},
"sales": {
"salesByCategory": [
{"categoryName": "DJ Cables", "totalSales": 1000},
{"categoryName": "DJ Headphones", "totalSales": 35921}
],
"fullSales": 3700
},
"promotionEvents": [
{
"eventName": "Cyber Monday Event",
"promotionalDates": {
"startDate": {"Year": 2024, "Month": 8, "Day": 1},
"endDate": {"Year": 2024, "Month": 8, "Day": 7}
},
"discounts": [{"categoryName": "DJ Speakers", "discountPercentage": 25}]
},
{
"eventName": "Black Friday Event",
"promotionalDates": {
"startDate": {"Year": 2024, "Month": 8, "Day": 1},
"endDate": {"Year": 2024, "Month": 8, "Day": 7}
},
"discounts": [{"categoryName": "DJ Speakers", "discountPercentage": 25}]
},
{
"eventName": "Mega Discount Days",
"promotionalDates": {
"startDate": {"Year": 2024, "Month": 5, "Day": 11},
"endDate": {"Year": 2024, "Month": 5, "Day": 18}
},
"discounts": [{"categoryName": "DJ Lights", "discountPercentage": 20}]
}
],
"tag": [
"#ShopLocal", "#NewArrival", "#FashionStore", "#SeasonalSale", "#FreeShipping", "#MembershipDeals"
]
}
该示例从搜索存储的 promotionEvents
数组中检索第一个事件详细信息。
db.stores.aggregate([
{ $match: { name: "Lakeshore Retail | DJ Equipment Stop - Port Cecile" } },
{
$project: {
firstPromotionEvent: { $arrayElemAt: ["$promotionEvents", 0] }
}
}
])
该查询返回查询存储的第一个升级事件 json。
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"firstPromotionEvent": {
"eventName": "Cyber Monday Event",
"promotionalDates": {
"startDate": { "Year": 2024, "Month": 8, "Day": 1 },
"endDate": { "Year": 2024, "Month": 8, "Day": 7 }
},
"discounts": [
{ "categoryName": "DJ Speakers", "discountPercentage": 25 }
]
}
}