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法律研究MongoDB 聚合以切片数组中的数组
为此，请在 MongoDB 中使用 aggregate()。在其中，使用 $slice 切片数组中的数组。让我们创建一个带有文档的集合 -
> db.demo111.insertOne(
... {
... "_id" : 101,
... "Name" : "Chris",
... "Details" : [
... {
... "_id" : 101,
... "Score" : 78,
... "Subjects" : [
... {
... "_id" : "10001",
... "SubjectName" : "MySQL"
... },
... {
... "_id" : "10003",
... "SubjectName" : "MongoDB"
... }
... ]
... },
... {
... "_id" : 102,
... "Score" : 87,
... "Subjects" : [
... {
... "_id" : "10004",
... "SubjectName" : "Java"
... }
... ]
... }
... ]
... }
... );
{ "acknowledged" : true, "insertedId" : 101 }使用 find() 方法从集合中显示所有文档 -
> db.demo111.find();
这将生成以下输出 -
{
"_id" : 101, "Name" : "Chris", "Details" : [
{ "_id" : 101, "Score" : 78, "Subjects" : [ { "_id" : "10001", "SubjectName" : "MySQL" },
{ "_id" : "10003", "SubjectName" : "MongoDB" } ] },
{ "_id" : 102, "Score" : 87, "Subjects" : [ { "_id" : "10004", "SubjectName" : "Java" } ] }
]
}以下是切片数组中数组的查询 -
> db.demo111.aggregate([
... { "$addFields": {
... "Details": {
... "$map": {
... "input": "$Details",
... "as": "out",
... "in": {
... "_id": "$$out._id",
... "Score": "$$out.Score",
... "Subjects": { "$slice": [ "$$out.Subjects", 1 ] }
... }
... }
... }
... }}
... ])这将生成以下输出 -
{
"_id" : 101, "Name" : "Chris", "Details" : [
{ "_id" : 101, "Score" : 78, "Subjects" : [ { "_id" : "10001", "SubjectName" : "MySQL" } ] },
{ "_id" : 102, "Score" : 87, "Subjects" : [ { "_id" : "10004", "SubjectName" : "Java" } ] }
]
}
更新于：30-Mar-2020
333 次浏览
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