$group by 之后的动态键 [英] Dynamic keys after $group by

问题描述

我有以下收藏

{
    "_id" : ObjectId("5b18d14cbc83fd271b6a157c"),
    "status" : "pending",
    "description" : "You have to complete the challenge...",
}
{
    "_id" : ObjectId("5b18d31a27a37696ec8b5773"),
    "status" : "completed",
    "description" : "completed...",
}
{
    "_id" : ObjectId("5b18d31a27a37696ec8b5775"),
    "status" : "pending",
    "description" : "pending...",
}
{
    "_id" : ObjectId("5b18d31a27a37696ec8b5776"),
    "status" : "inProgress",
    "description" : "inProgress...",
}

我需要按 status 分组并动态获取 status

I need to group by status and get all the keys dynamically which are in status

[
  {
    "completed": [
      {
        "_id": "5b18d31a27a37696ec8b5773",
        "status": "completed",
        "description": "completed..."
      }
    ]
  },
  {
    "pending": [
      {
        "_id": "5b18d14cbc83fd271b6a157c",
        "status": "pending",
        "description": "You have to complete the challenge..."
      },
      {
        "_id": "5b18d31a27a37696ec8b5775",
        "status": "pending",
        "description": "pending..."
      }
    ]
  },
  {
    "inProgress": [
      {
        "_id": "5b18d31a27a37696ec8b5776",
        "status": "inProgress",
        "description": "inProgress..."
      }
    ]
  }
]

推荐答案

并不是我认为这是一个好主意，主要是因为我在这里根本看不到任何聚合"是在分组"之后添加到数组你同样 $push all该内容通过 "status" 分组键转换为数组，然后在 $replaceRoot 与 $arrayToObject:

Not that I think it's a good idea and mostly because I don't see any "aggregation" here at all is that after "grouping" to add to an array you similarly $push all that content into array by the "status" grouping key and then convert into keys of a document in a $replaceRoot with $arrayToObject:

db.collection.aggregate([
  { "$group": {
    "_id": "$status",
    "data": { "$push": "$$ROOT" }
  }},
  { "$group": {
    "_id": null,
    "data": {
      "$push": {
        "k": "$_id",
        "v": "$data"
      }
    }
  }},
  { "$replaceRoot": {
    "newRoot": { "$arrayToObject": "$data" }
  }}
])

返回:

{
        "inProgress" : [
                {
                        "_id" : ObjectId("5b18d31a27a37696ec8b5776"),
                        "status" : "inProgress",
                        "description" : "inProgress..."
                }
        ],
        "completed" : [
                {
                        "_id" : ObjectId("5b18d31a27a37696ec8b5773"),
                        "status" : "completed",
                        "description" : "completed..."
                }
        ],
        "pending" : [
                {
                        "_id" : ObjectId("5b18d14cbc83fd271b6a157c"),
                        "status" : "pending",
                        "description" : "You have to complete the challenge..."
                },
                {
                        "_id" : ObjectId("5b18d31a27a37696ec8b5775"),
                        "status" : "pending",
                        "description" : "pending..."
                }
        ]
}

这可能没关系如果你实际上是事先聚合"的，但在任何实际大小的集合上，所做的只是试图将整个集合强制转换为单个文档，这可能会破坏 BSON 限制为 16MB，所以我什至不建议尝试这个在这一步之前没有分组"其他东西.

That might be okay IF you actually "aggregated" beforehand, but on any practically sized collection all that is doing is trying force the whole collection into a single document, and that's likely to break the BSON Limit of 16MB, so I just would not recommend even attempting this without "grouping" something else before this step.

坦率地说，以下相同的代码做同样的事情，并且没有聚合技巧，也没有 BSON 限制问题:

Frankly, the same following code does the same thing, and without aggregation tricks and no BSON limit problem:

var obj = {};

// Using forEach as a premise for representing "any" cursor iteration form
db.collection.find().forEach(d => {
  if (!obj.hasOwnProperty(d.status))
    obj[d.status] = [];
  obj[d.status].push(d);
})

printjson(obj);

或者更短一点:

var obj = {};

// Using forEach as a premise for representing "any" cursor iteration form
db.collection.find().forEach(d => 
  obj[d.status] = [ 
    ...(obj.hasOwnProperty(d.status)) ? obj[d.status] : [],
    d
  ]
)

printjson(obj);

聚合用于数据减少"，任何只是重塑结果"而没有实际减少从服务器返回的数据的东西通常最好在客户端代码中处理.无论您做什么，您仍然会返回所有数据，并且游标的客户端处理的开销要少得多.并且没有任何限制.

Aggregations are used for "data reduction" and anything that is simply "reshaping results" without actually reducing the data returned from the server is usually better handled in client code anyway. You're still returning all data no matter what you do, and the client processing of the cursor has considerably less overhead. And NO restrictions.

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