从MongoDB find()结果集中确定最后一个文档？
要从MongoDBfind()结果集中识别最后一个文档，可以sort()按降序使用。语法如下-
db.yourCollectionName.find().sort( { _id : -1 } ).limit(1).pretty();为了理解上述语法，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.identifyLastDocuementDemo.insertOne({"UserName":"Larry","UserAge":24,"UserCountryName":"US"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a2ff4cf1f7a64fa4df57")
}
> db.identifyLastDocuementDemo.insertOne({"UserName":"Chris","UserAge":21,"UserCountryName":"UK"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a3094cf1f7a64fa4df58")
}
> db.identifyLastDocuementDemo.insertOne({"UserName":"David","UserAge":25,"UserCountryName":"AUS"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a3174cf1f7a64fa4df59")
}
> db.identifyLastDocuementDemo.insertOne({"UserName":"Sam","UserAge":26,"UserCountryName":"US"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a3224cf1f7a64fa4df5a")
}
> db.identifyLastDocuementDemo.insertOne({"UserName":"Mike","UserAge":27,"UserCountryName":"AUS"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a32e4cf1f7a64fa4df5b")
}
> db.identifyLastDocuementDemo.insertOne({"UserName":"Carol","UserAge":28,"UserCountryName":"UK"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c94a33c4cf1f7a64fa4df5c")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.identifyLastDocuementDemo.find().pretty();
以下是输出-
{
"_id" : ObjectId("5c94a2ff4cf1f7a64fa4df57"),
"UserName" : "Larry",
"UserAge" : 24,
"UserCountryName" : "US"
}
{
"_id" : ObjectId("5c94a3094cf1f7a64fa4df58"),
"UserName" : "Chris",
"UserAge" : 21,
"UserCountryName" : "UK"
}
{
"_id" : ObjectId("5c94a3174cf1f7a64fa4df59"),
"UserName" : "David",
"UserAge" : 25,
"UserCountryName" : "AUS"
}
{
"_id" : ObjectId("5c94a3224cf1f7a64fa4df5a"),
"UserName" : "Sam",
"UserAge" : 26,
"UserCountryName" : "US"
}
{
"_id" : ObjectId("5c94a32e4cf1f7a64fa4df5b"),
"UserName" : "Mike",
"UserAge" : 27,
"UserCountryName" : "AUS"
}
{
"_id" : ObjectId("5c94a33c4cf1f7a64fa4df5c"),
"UserName" : "Carol",
"UserAge" : 28,
"UserCountryName" : "UK"
}这是从MongoDBfind()结果集中识别最后一个文档的查询-
> db.identifyLastDocuementDemo.find().sort( { _id : -1 } ).limit(1).pretty();以下是输出-
{
"_id" : ObjectId("5c94a33c4cf1f7a64fa4df5c"),
"UserName" : "Carol",
"UserAge" : 28,
"UserCountryName" : "UK"
}