聚合 $graphLookup 每次执行查询时都以不同的顺序检索结果？

Question

我有以下数据集（类别）：

[{
  "_id": 1,
  "name": "Root",
  "parent": null
},
 {
   "_id": 2,
   "name": "Sub - Level 1",
   "parent": 1
 }, {
   "_id": 3,
   "name": "Sub - Level 2",
   "parent": 2
 }, {
   "_id": 4,
   "name": "Sub - Level 3",
   "parent": 3
 }
]

我是运行此数据集上的以下管道以递归获取树：

[{
  '$match': {
    '_id': 1
  }
}, {
  '$graphLookup': {
    'from': 'categories', 
    'startWith': '$_id', 
    'connectFromField': '_id', 
    'connectToField': 'parent', 
    'as': 'menu'
  }
}, {
    '$sort': {
      'menu.parent': 1
    }
  }]

目的是像这样递归地获取树：

{
    "_id": 1,
    "name": "Root",
    "parent: "null",
    "menu": [
        {..sub},{..sub},{...sub}
    ]
}

它完成了工作，但每次执行查询时，menu 数组中元素的顺序都不同。每次！

"menu" : [{... Sub - Level 2},{... Sub - Level 3}, {... Sub - Level 1}]
"menu" : [{... Sub - Level 1},{... Sub - Level 3}, {... Sub - Level 2}]
"menu" : [{... Sub - Level 3},{... Sub - Level 1}, {... Sub - Level 2}]

这是 $graphLookup 的正常行为还是我错过了什么？我应该如何对 menu 数组进行排序？

Answer 1

Documents returned in the as field are not guaranteed to be in any order.

https://docs.mongodb.com/manual/reference/operator/aggregation/graphLookup/#definition

If you need an order, consider specifying the 'depthField' argument, and sorting by that.

https://jira.mongodb.org/browse/SERVER-26153

解决方法：

db.categories.aggregate([
  {
    "$match": {
      "_id": 1
    }
  },
  {
    "$graphLookup": {
      "from": "categories",
      "startWith": "$_id",
      "connectFromField": "_id",
      "connectToField": "parent",
      "as": "menu",
      depthField: "order"
    }
  },
  {
    $unwind: "$menu"
  },
  {
    $sort: {
      _id: 1,
      "menu.order": 1
    }
  },
  {
    $group: {
      _id: "$_id",
      name: {
        $first: "$name"
      },
      parent: {
        $first: "$parent"
      },
      menu: {
        $push: "$menu"
      }
    }
  },
  {
    $unset: "menu.order" //Change to $project:{"menu.order":0} for MongoDB <v4.2
  }
])

MongoPlayground

聚合 $graphLookup 每次执行查询时都以不同的顺序检索结果？

Aggregation $graphLookup retrieving results in different order every-time the query is executed?

mongodb

mongoose

mongodb-query

aggregation-framework

graphlookup