如何在所有MongoDB文档中查找数组中的最新条目？
要在所有文档中查找数组中的最新条目，请使用aggregate()。让我们创建一个包含文档的集合-
> db.demo179.insertOne(
...{
... "Name":"Chris",
... "Details": [
... {
... "Id":101,
... "Subject":"MongoDB"
... },
... {
... "Id":102,
... "Subject":"MySQL"
... }
... ]
...}
...);
{
"acknowledged" : true,
"insertedId" : ObjectId("5e3980299e4f06af551997f9")
}
> db.demo179.insertOne(
...{
... "Name":"David",
... "Details": [
... {
... "Id":103,
... "Subject":"Java"
... },
... {
... "Id":104,
... "Subject":"C"
... }
... ]
...}
...);
{
"acknowledged" : true,
"insertedId" : ObjectId("5e39802a9e4f06af551997fa")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo179.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5e3980299e4f06af551997f9"),
"Name" : "Chris",
"Details" : [
{
"Id" : 101,
"Subject" : "MongoDB"
},
{
"Id" : 102,
"Subject" : "MySQL"
}
]
}
{
"_id" : ObjectId("5e39802a9e4f06af551997fa"),
"Name" : "David",
"Details" : [
{
"Id" : 103,
"Subject" : "Java"
},
{
"Id" : 104,
"Subject" : "C"
}
]
}以下是在所有文档中查找数组中最新条目的查询-
> db.demo179.aggregate([
... { "$unwind": "$Details" },
... { "$sort": { "Details.Id": -1 } },
... { "$limit": 2 },
... {
... "$group": {
... "_id": "$Details.Id",
... "Name" : { "$first": "$Name" },
... "Details": { "$push": "$Details" }
... }
... },
... {
... "$project": {
... "_id": 0, "Name": 1, "Details": 1
... }
... }
...])这将产生以下输出-
{ "Name" : "David", "Details" : [ { "Id" : 103, "Subject" : "Java" } ] }
{ "Name" : "David", "Details" : [ { "Id" : 104, "Subject" : "C" } ] }