MongoDB Aggregate-查询以获取组中的最新项目 [英] MongoDB Aggregate - Query to get most recent item in group

问题描述

给出此源数据:

{ "_id" : ObjectId("1"), "productID" : 1, "amount" : 1, "date" : ISODate("2017-02-01T00:00:00Z") }
{ "_id" : ObjectId("2"), "productID" : 2, "amount" : 2, "date" : ISODate("2017-02-01T00:00:00Z") }
{ "_id" : ObjectId("3"), "productID" : 3, "amount" : 3, "date" : ISODate("2017-02-01T00:00:00Z") }
{ "_id" : ObjectId("4"), "productID" : 4, "amount" : 4, "date" : ISODate("2017-02-01T00:00:00Z") }
{ "_id" : ObjectId("5"), "productID" : 1, "amount" : 11, "date" : ISODate("2017-02-02T00:00:00Z") }
{ "_id" : ObjectId("6"), "productID" : 2, "amount" : 22, "date" : ISODate("2017-02-02T00:00:00Z") }
{ "_id" : ObjectId("7"), "productID" : 2, "amount" : 222, "date" : ISODate("2017-02-03T00:00:00Z") }
{ "_id" : ObjectId("8"), "productID" : 3, "amount" : 33, "date" : ISODate("2017-02-03T00:00:00Z") }

我想获取每种产品的最新记录(在productID上键入)，并为在productID上排序的每种产品打印出该记录的整行.因此，我希望从上面得到的输出是:

I want to get the most recent record for each product (keyed on productID) and print out the full row for that record per product sorted on productID. So my desired output from above would be:

{ "_id" : ObjectId("5"), "productID" : 1, "amount" : 11, "date" : ISODate("2017-02-02T00:00:00Z") }
{ "_id" : ObjectId("7"), "productID" : 2, "amount" : 222, "date" : ISODate("2017-02-03T00:00:00Z") }
{ "_id" : ObjectId("8"), "productID" : 3, "amount" : 33, "date" : ISODate("2017-02-03T00:00:00Z") }
{ "_id" : ObjectId("4"), "productID" : 4, "amount" : 4, "date" : ISODate("2017-02-01T00:00:00Z") }

我是Mongo的新手，遇到了问题，设法解决了大部分问题，但无法删除每种产品的其他较早的记录.

I'm new to Mongo and having problems, managed to get most of it but couldn't delete the other older records for each product.

推荐答案

您可以使用 $ first 运算符.请注意，您应该在分组之前对文档进行排序:

You can use $last and $first operators. Note that you should sort documents before grouping:

db.so.aggregate([
 { $sort: {productID:-1, date: -1} },
 { 
     $group: {
         _id : "$productID",
         date: {$last: "$date" },
         amount: {$first: "$amount" },
         id : {$first: "$_id"}
     }
 },
 { $project: { _id: "$id", productId: "$_id", date: 1, amount: 1 } }
])

输出:

{
    "_id" : 5,
    "productId" : 1,
    "date" : ISODate("2017-02-01T00:00:00.000Z"),
    "amount" : 11
},
{
    "_id" : 7,
    "productId" : 2,
    "date" : ISODate("2017-02-01T00:00:00.000Z"),
    "amount" : 222
},
{
    "_id" : 8,
    "productId" : 3,
    "date" : ISODate("2017-02-01T00:00:00.000Z"),
    "amount" : 33
},
{
    "_id" : 4,
    "productId" : 4,
    "date" : ISODate("2017-02-01T00:00:00.000Z"),
    "amount" : 4
}

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