如何用数组中的多键索引改进MongoDB查询？
为此，请使用$elemMatch，该查询用于查询嵌套对象。让我们创建一个包含文档的集合-
> db.demo444.insertOne(
... {
... "Information": [{
... id:1,
... Name:"Chris"
... }]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e78ea87bbc41e36cc3caebf")
}
> db.demo444.insertOne(
... {
... "Information": [{
... id:2,
... Name:"David"
... }]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e78ea87bbc41e36cc3caec0")
}
> db.demo444.insertOne(
... {
... "Information": [{
... id:3,
... Name:"Bob"
... }]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e78ea88bbc41e36cc3caec1")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo444.find();
这将产生以下输出-
{ "_id" : ObjectId("5e78ea87bbc41e36cc3caebf"), "Information" : [ { "id" : 1, "Name" : "Chris" } ] }
{ "_id" : ObjectId("5e78ea87bbc41e36cc3caec0"), "Information" : [ { "id" : 2, "Name" : "David" } ] }
{ "_id" : ObjectId("5e78ea88bbc41e36cc3caec1"), "Information" : [ { "id" : 3, "Name" : "Bob" } ] }以下是使用数组中的多键索引改进查询的查询-
> db.demo444.find({
... "Information":{
... $elemMatch:{
... id:2,
... Name:"David"
... }
... }
... })这将产生以下输出-
{ "_id" : ObjectId("5e78ea87bbc41e36cc3caec0"), "Information" : [ { "id" : 2, "Name" : "David" } ] }