MongoDB - 查询数组的最后一个元素？
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MongoDB - Query on the last element of an array?
提问by Joseph Blair
I know that MongoDB supports the syntax find{array.0.field:"value"}, but I specifically want to do this for the last element in the array, which means I don't know the index. Is there some kind of operator for this, or am I out of luck?
我知道 MongoDB 支持语法find{array.0.field:"value"}，但我特别想对数组中的最后一个元素执行此操作，这意味着我不知道索引。是否有某种运算符，或者我不走运？
EDIT: To clarify, I want find() to only return documents where a field in the last element of an array matches a specific value.
编辑：为了澄清，我希望 find() 仅返回数组最后一个元素中的字段与特定值匹配的文档。
采纳答案by Joseph Blair
回答by jdeyrup
In 3.2 this is possible. First project so that myField contains only the last element, and then match on myField.
在 3.2 中这是可能的。第一个项目使 myField 只包含最后一个元素，然后在 myField 上进行匹配。
db.collection.aggregate([
{ $project: { id: 1, myField: { $slice: [ "$myField", -1 ] } } },
{ $match: { myField: "myValue" } }
]);
回答by Sagar Veeram
You can use $expr( 3.6 mongo version operator ) to use aggregation functions in regular query.
您可以使用$expr( 3.6 mongo version operator ) 在常规查询中使用聚合函数。
Compare query operatorsvs aggregation comparison operators.
比较query operatorsvs aggregation comparison operators。
For scalar arrays
对于标量数组
db.col.find({$expr: {$gt: [{$arrayElemAt: ["array", -1]}, value]}})
For embedded arrays - Use $arrayElemAtexpression with dot notation to project last element.
对于嵌入式数组 - 使用$arrayElemAt带点符号的表达式来投影最后一个元素。
db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})
Spring @Querycode
弹簧@Query代码
@Query("{$expr:{$gt:[{$arrayElemAt:[\"array\", -1]}, ?0]}}")
ReturnType MethodName(ArgType arg);
回答by Kaushal
use $slice.
使用$slice。
db.collection.find( {}, { array_field: { $slice: -1 } } )
Editing:
You can make use of
{ <field>: { $elemMatch: { <query1>, <query2>, ... } } }to find a match.
编辑：您可以使用
{ <field>: { $elemMatch: { <query1>, <query2>, ... } } }来查找匹配项。
But it won't give exactly what you are looking for. I don't think that is possible in mongoDB yet.
但它不会给出你正在寻找的东西。我认为这在 mongoDB 中是不可能的。
回答by Ramil Muratov
Not sure about performance, but this works well for me:
不确定性能，但这对我很有效：
db.getCollection('test').find(
{
$where: "this.someArray[this.someArray.length - 1] === 'pattern'"
}
)
回答by KARTHIKEYAN.A
Version 3.6use aggregation to achieve the same.
3.6 版使用聚合来实现相同的功能。
db.getCollection('deviceTrackerHistory').aggregate([
{
$match:{clientId:"12"}
},
{
$project:
{
deviceId:1,
recent: { $arrayElemAt: [ "$history", -1 ] }
}
}
])
回答by Ricky
You could use $position: 0whenever you $push, and then always query array.0to get the most recently added element. Of course then, you wont be able to get the new "last" element.
你可以使用$position: 0，只要你$push，然后总是查询array.0以获得最近添加的元素。当然，您将无法获得新的“最后”元素。
回答by Xavier Guihot
Starting Mongo 4.4, the aggregation operator $lastcan be used to access the last element of an array:
从 开始Mongo 4.4，聚合运算符$last可用于访问数组的最后一个元素：
For instance, within a findquery:
例如，在find查询中：
// { "myArray": ["A", "B", "C"] }
// { "myArray": ["D"] }
db.collection.find({ $expr: { $eq: [{ $last: "$myArray" }, "C"] } })
// { "myArray": ["A", "B", "C"] }
Or within an aggregationquery:
或在aggregation查询中：
db.collection.aggregate([
{ $addFields: { last: { $last: "$myArray" } } },
{ $match: { last: "C" } }
])
回答by Robin Jiao
For the answer use $arrayElemAt,if i want orderNumber:"12345" and the last element's value $gt than "value"? how to make the $expr? thanks！
对于答案使用$arrayElemAt，如果我想要 orderNumber:"12345" 和最后一个元素的值 $gt 而不是 "value"？如何制作$expr？谢谢！
For embedded arrays - Use $arrayElemAtexpression with dot notation to project last element.
对于嵌入式数组 - 使用$arrayElemAt带点符号的表达式来投影最后一个元素。
db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})
