$group 和时间戳 $sort mongodb答案

【问题标题】：$group and timestamp $sort mongodb$group 和时间戳 $sort mongodb
【发布时间】：2017-07-01 10:28:20
【问题描述】：

我想按时间戳对文档进行排序并获得不同的孩子姓名。所以查询的预期是这样的；

[alex, elizabeth, felix, tom, sonny, ashton, sharon]

集合是；

{ "timestamp" : 1486641003000, children: [{name: "sharon"}] },
{ "timestamp" : 1486645481504, children: [{name: "tom"} , {name: "alex"}]},
{ "timestamp" : 1486732863102, children: [{name: "alex"}, {name: "elizabeth"}] },
{ "timestamp" : 1486642403974, children: [{name: "sonny"}] },
{ "timestamp" : 1486641003000, children: [{name: "elizabeth"}] },
{ "timestamp" : 1486645481504, children: [{name: "felix"}] },
{ "timestamp" : 1486645481504, children: [{name: "tom"} , {name: "alex"}]},
{ "timestamp" : 1486642623178, children: [{name: "alex"}] },
{ "timestamp" : 1486642403974, children: [{name: "felix"}] },
{ "timestamp" : 1486641003000, children: [{name: "ashton"}] },

到目前为止我做了什么？

db.collection.aggregate([
    {$unwind: "$children"},
    {$sort: {"timestamp" : -1}},
    {$group: {"_id" : "" , children: {$addToSet: "$children.name"}}}
])

但我认为分组会根据时间戳覆盖排序。因为结果和我预想的不一样；

{ "_id" : "", "children" : [ "ashton", "felix", "tom", "sonny", "elizabeth", "sharon", "alex" ] }

那么在按时间戳对孩子进行排序时，我如何才能获得最后更改的不同孩子的姓名。

【问题讨论】：

标签： mongodb sorting grouping

【解决方案1】：

因此，在这种情况下，您将不得不再进行一次排序和分组。

db.collection.aggregate([
    {$unwind: "$children"}, 
    {$sort: { "timestamp": -1 }},
    {$group: {"_id": "$children.name", "timestamp": {$first: "$timestamp"}}},
    {$sort: { "timestamp": -1 }}, 
    {$group: {"_id": null, "children": {$push: "$_id"}}}
])

结果将如下所示：

{ "_id" : null, "children" : [ "alex", "elizabeth", "tom", "felix", "sonny", "sharon", "ashton" ] }

这是因为“tom”和“felix”的时间戳相似，与“sharon”和“ashton”相同。

【讨论】：