MongoDB如何根据文档与整个集合的比较情况提取整个文档
为此，请在MongoDB中使用$$ROOT。让我们创建一个包含文档的集合-
> db.demo743.insertOne({id:1,"ShippingDate":"2020-01-21",value:50});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ead893a57bb72a10bcf0680")
}
> db.demo743.insertOne({id:2,"ShippingDate":"2020-05-10",value:30});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ead893c57bb72a10bcf0681")
}
> db.demo743.insertOne({id:3,"ShippingDate":"2020-05-10",value:60});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ead894657bb72a10bcf0682")
}
> db.demo743.insertOne({id:1,"ShippingDate":"2020-05-11",value:75});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ead895657bb72a10bcf0683")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo743.find();
这将产生以下输出-
{ "_id" : ObjectId("5ead893a57bb72a10bcf0680"), "id" : 1, "ShippingDate" : "2020-01-21", "value" : 50 }
{ "_id" : ObjectId("5ead893c57bb72a10bcf0681"), "id" : 2, "ShippingDate" : "2020-05-10", "value" : 30 }
{ "_id" : ObjectId("5ead894657bb72a10bcf0682"), "id" : 3, "ShippingDate" : "2020-05-10", "value" : 60 }
{ "_id" : ObjectId("5ead895657bb72a10bcf0683"), "id" : 1, "ShippingDate" : "2020-05-11", "value" : 75 }以下是根据它们与整个集合进行比较的方式提取整个文档的查询-
> db.demo743.aggregate([
... {
... "$project":
... {
... "id": "$id",
... "ShippingDate": "$ShippingDate",
... "MyDoc": "$$ROOT"
... }
... },
... {
... "$sort":
... { "ShippingDate": -1
... }
... },
... {
... "$group":
... {
... "_id":{"id":"$id"},
... "Result":{"$first":"$MyDoc"}
... }
... },
... {
... $project:
... {
... "Result.ShippingDate":1, "Result.id":1, "Result.value":1, _id:0}
... }
... ])这将产生以下输出-
{ "Result" : { "id" : 3, "ShippingDate" : "2020-05-10", "value" : 60 } }
{ "Result" : { "id" : 2, "ShippingDate" : "2020-05-10", "value" : 30 } }
{ "Result" : { "id" : 1, "ShippingDate" : "2020-05-11", "value" : 75 } }