Mongo 聚合每组文档的 returns 最后版本

Question

给定名为 sampledata 的集合中的以下文档：

sampledata:
{
    "link" : "demo-1",
    "type" : "type-1",
    "version" : 1,
    "slug" : "slug-1",
    "_id" : ObjectId("550acc38fd15150c290dd489")
}
{
    "link" : "demo-2",
    "type" : "type-2",
    "version" : 1,
    "slug" : "slug-2",
    "_id" : ObjectId("550acc37fd15150c290dd46a")
}
{
    "link" : "demo-3",
    "type" : "type-3",
    "version" : 1,
    "slug" : "slug-3",
    "_id" : ObjectId("550acc38fd15150c290dd46e")
}
{
    "link" : "demo-3",
    "type" : "type-3",
    "version" : 2,
    "slug" : "slug-3",
    "_id" : ObjectId("550b6182dac1909834b7b38d")
}
{
    "link" : "demo-3",
    "type" : "type-3",
    "version" : 3,
    "slug" : "slug-3",
    "_id" : ObjectId("550b61d3dac1909834b7b38e")
}

我需要一个 returns 每个 "type" 文档的最后一个 "version" 的聚合管道。因此，结果应该是：

{
    "link" : "demo-1",
    "type" : "type-1",
    "version" : 1,
    "slug" : "slug-1",
    "_id" : ObjectId("550acc38fd15150c290dd489")
}
{
    "link" : "demo-2",
    "type" : "type-2",
    "version" : 1,
    "slug" : "slug-2",
    "_id" : ObjectId("550acc37fd15150c290dd46a")
}
{
    "link" : "demo-3",
    "type" : "type-3",
    "version" : 3,
    "slug" : "slug-3",
    "_id" : ObjectId("550b61d3dac1909834b7b38e")
}

以下提供了我想要的大部分内容，但我很难按原样返回所选文档中的其余字段：

db.sampledata.aggregate([{"$group": { "_id": "$slug", lastVersion: {"$last":"$version"} } }])

Answer 1

$last operator works in the "last" item in a grouping boundary from the supplied key. Typically it is used with $sort preceeding the following $group语句。这 "orders" 集合中的文档以所需的方式，以便 "last" 获取预期的内容：

db.sampledata.aggregate([
    { "$sort": { "slug": 1, "version": 1 } },
    { "$group": {
        "_id": "$slug",
        "link": { "$last": "$link" },
        "type": { "$last": "$type" },
        "version": { "$last": "$version" },
        "id": { "$last": "$_id" }
    }}
])

如您所见，如果您想匹配按该顺序 "last" 的整个文档，其目的是对每个字段使用运算符。要完全重建，您需要以下 $project:

db.sampledata.aggregate([
    { "$sort": { "slug": 1, "version": 1 } },
    { "$group": {
        "_id": "$slug",
        "link": { "$last": "$link" },
        "type": { "$last": "$type" },
        "version": { "$last": "$version" },
        "id": { "$last": "$_id" }
    }},
    { "$project": {
        "_id": "$id",
        "slug": "$_id",
        "link": 1,
        "type": 1,
        "version": 1
    }}
])

Mongo 聚合每组文档的 returns 最后版本

Mongo aggregate that returns last version of each set of documents

mongodb

mongodb-query

aggregation-framework