MongoDB聚合，获取一个值数组并获取它们的数量

Question

这是我第一次在StackOverflow中提问，我希望我能解释我的目标。我有这样的文件：

"_id" : ObjectId("5fd76b67a7e0fa652a297a9f"),
"type" : "play",
"session" : "5b0b5d57-c3ca-415f-8ef6-49bbd5805a23",
"episode" : 1,
"show" : 1,
"user" : 1,
"platform" : "spotify",
"currentTime" : 0,
"date" : ISODate("2020-12-14T13:40:51.906Z"),
"__v" : 0
}

我想找个节目，按集分组。我的聚合已经走到了这一步：

const filter = { user, show, type: { $regex: /^(play|stop|close)$/ } }
const requiredFields = { "episode": 1, "session": 1, "date": 1, "currentTime": 1 }
// Get sessions grouped by episode
const it0 = {
_id: '$episode',
session:
{$addToSet:
{_id: "$session",
date:{$dateToString: { format: "%Y-%m-%d", date: "$date" }},
averageOfSession: {$cond: [ { $gte: [ "$currentTime", 0.1 ] }, "$currentTime", null ] }
},
},
count: { $sum: 1 }
}
// Filter unique sessions by session id and add them to a sessions field
const reduceSessions = {$addFields:
{sessions: {$reduce: {input: "$session",initialValue: [],in:
{$concatArrays: ["$$value",{$cond: [{$in: ["$$this._id","$$value._id"]},[],["$$this"]]}]}
}}}}
const projection = { $project: { _id: 0, episode: "$_id", plays: {$size: '$sessions'}, dropoff: {$avg: "$sessions.averageOfSession"}, sessions: '$session.date', events: "$count" } }
const arr = await Play.aggregate([
{ $match: filter }, {$project: requiredFields}, {$group: it0}, reduceSessions,
projection,{ $sort : { _id : 1 } }
])

这就是我目前的结果：

{
"episode": 5,
"plays": 4,
"dropoff": 3737.25,
"sessions": [
"2020-11-15",
"2020-11-15",
"2020-11-16",
"2020-11-15"
],
"events": 4
}...

我希望"sessions"数组是一个对象，每个不同的日期都有一个键，其中包含计数，所以类似这样的东西：

{
"episode": 5,
"plays": 4,
"dropoff": 3737.25,
"sessions": {
"2020-11-15": 3,
"2020-11-16": 1
},
"events": 4
}...

希望这有意义，谢谢！！

Answer 1

您可以首先将sessions映射到键值对中。然后$将它们分组，将总数相加。然后使用$arrayToObject将其转换为所需的格式。

这个Mongo游乐场引用了这个例子。