适用对象：
MongoDB vCore
该 $lastN 运算符返回数组中的最后 n 个元素。 如果要限制从数组末尾返回的元素数，例如从按时间顺序排序的列表获取最新项，则它很有用。
语法
$lastN 运算符的语法如下：
{
$lastN: {
input: <array>,
n: <number>
}
}
参数
| DESCRIPTION | |
|---|---|
input |
要从中返回最后 n 个元素的数组。 |
n |
要从数组末尾返回的元素数。 必须是正整数。 |
示例：
让我们了解 stores 数据集中的示例 json 的用法。
{
"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
"name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
"promotionEvents": [
{
"eventName": "Massive Markdown Mania",
"promotionalDates": {
"startDate": { "Year": 2023, "Month": 6, "Day": 29 },
"endDate": { "Year": 2023, "Month": 7, "Day": 9 }
}
},
{
"eventName": "Fantastic Deal Days",
"promotionalDates": {
"startDate": { "Year": 2023, "Month": 9, "Day": 27 },
"endDate": { "Year": 2023, "Month": 10, "Day": 7 }
}
},
{
"eventName": "Discount Delight Days",
"promotionalDates": {
"startDate": { "Year": 2023, "Month": 12, "Day": 26 },
"endDate": { "Year": 2024, "Month": 1, "Day": 5 }
}
},
{
"eventName": "Super Sale Spectacular",
"promotionalDates": {
"startDate": { "Year": 2024, "Month": 3, "Day": 25 },
"endDate": { "Year": 2024, "Month": 4, "Day": 2 }
}
},
{
"eventName": "Grand Deal Days",
"promotionalDates": {
"startDate": { "Year": 2024, "Month": 6, "Day": 23 },
"endDate": { "Year": 2024, "Month": 6, "Day": 30 }
}
},
{
"eventName": "Major Bargain Bash",
"promotionalDates": {
"startDate": { "Year": 2024, "Month": 9, "Day": 21 },
"endDate": { "Year": 2024, "Month": 9, "Day": 30 }
}
}
]
}
示例 1：获取最后两个促销事件
假设你想要从一家商店获取最近的两个促销活动。
db.stores.aggregate([
{ $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
{
$project: {
name: 1,
lastTwoPromotions: {
$lastN: {
input: "$promotionEvents",
n: 2
}
}
}
}
])
这会生成以下输出：
[
{
_id: '40d6f4d7-50cd-4929-9a07-0a7a133c2e74',
name: 'Proseware, Inc. | Home Entertainment Hub - East Linwoodbury',
lastTwoPromotions: [
{
eventName: 'Grand Deal Days',
promotionalDates: {
startDate: { Year: 2024, Month: 6, Day: 23 },
endDate: { Year: 2024, Month: 6, Day: 30 }
},
discounts: [
{ categoryName: 'Remote Controls', discountPercentage: 7 },
{ categoryName: 'Televisions', discountPercentage: 11 },
{
categoryName: 'Business Projectors',
discountPercentage: 13
},
{ categoryName: 'Laser Projectors', discountPercentage: 6 },
{ categoryName: 'Projectors', discountPercentage: 6 },
{ categoryName: 'Projector Screens', discountPercentage: 24 }
]
},
{
eventName: 'Major Bargain Bash',
promotionalDates: {
startDate: { Year: 2024, Month: 9, Day: 21 },
endDate: { Year: 2024, Month: 9, Day: 30 }
},
discounts: [
{ categoryName: 'Sound Bars', discountPercentage: 9 },
{ categoryName: 'VR Games', discountPercentage: 7 },
{ categoryName: 'Xbox Games', discountPercentage: 25 },
{
categoryName: 'Projector Accessories',
discountPercentage: 18
},
{ categoryName: 'Mobile Games', discountPercentage: 8 },
{ categoryName: 'Projector Cases', discountPercentage: 22 }
]
}
]
}
]
示例 2：获取最后三个销售类别
还可以用于 $lastN 获取最后几个销售类别。
db.stores.aggregate([
{ $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
{
$project: {
name: 1,
lastThreeCategories: {
$lastN: {
input: "$sales.salesByCategory",
n: 3
}
}
}
}
])
这会从 salesByCategory 数组返回最后三个销售类别。
[
{
_id: '40d6f4d7-50cd-4929-9a07-0a7a133c2e74',
name: 'Proseware, Inc. | Home Entertainment Hub - East Linwoodbury',
lastThreeCategories: [
{ categoryName: 'Game Controllers', totalSales: 43522 },
{ categoryName: 'Remote Controls', totalSales: 28946 },
{ categoryName: 'VR Games', totalSales: 32272 }
]
}
]