Question

如何获得最大的sections.Id在下面的文档中collection._id = some parameter

{
    "_id" : ObjectId("571c5c87faf473f40fd0317c"),
    "name" : "test 1",
    "sections" : [ 
        {
             "Id" : 1,
             "name" : "first section"
        }, 
        {
            "Id" : 2,
            "name" : "section 2"
        }, 
        {
            "Id" : 3,
            "name" : "section 3"
        }
}

我在下面试过

db.collection.aggregate(
[
    {
        "$match": {
            "_id": ObjectId("571c5c87faf473f40fd0317c")
        }
    },
    {
        "$group" : {
            "_id" : "$_id",
            "maxSectionId" : {"$max" : "$sections.Id"}
        }
    }
]);

但是不是返回max int单值，而是返回sections数组中所有Ids的数组。

在node.js中执行时，同样的查询返回一个空数组。

Answer 1

对于opennig to package Hierarchies; public static void generateHierarchy() { Set<Entry<Long, Node>> iterator = graph.entrySet(); int i =0; for(Entry<Long,Node> e : iterator) { System.out.println(i++ +" - " +e.getValue().firstParents()); } } @SuppressWarnings({ "resource" }) public static void main(String[] args) throws JWNLException { // TODO Auto-generated method stub dictionary = Dictionary.getDefaultResourceInstance(); File dir = new File("C:/Users/D060891/Desktop/Thesis/sentencesNYT"); for (File file : dir.listFiles()) { try { BufferedReader input = new BufferedReader(new FileReader(file)); String line; while ((line = input.readLine()) != null) { demonstrateListHelper(line); } } catch (IOException e) { e.printStackTrace(); } } generateHierarchy(); PrintStream out; try { out = new PrintStream(new FileOutputStream("output.txt")); System.setOut(out); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } } } array

，您的聚合查询需要

From nginx
Then change config

Cmd["nginx"]

添加您的聚合查询

$unwind

和您的重构聚合查询一样

"sections"

以及{$unwind : "$sections"}的更多知识：https://docs.mongodb.org/manual/reference/operator/aggregation/unwind/

Answer 2

您可以使用简单的$project阶段

类似this

db.collection.aggregate([
  { "$project": {
    "maxSectionId": {
      "$arrayElemAt": [
        "$sections",
        {
          "$indexOfArray": [
            "$sections.Id",
            { "$max": "$sections.Id" }
          ]
        }
      ]
    }
  }}
])

Answer 3

将$ group替换为$ project

In the $group stage, if the expression resolves to an array, $max does not traverse the array and compares the array as a whole.

With a single expression as its operand, if the expression resolves to an array, $max traverses into the array to operate on the numerical elements of the array to return a single value [sic]

MongoDb查询获取数组内的最大字段

3 个答案: