使用 MongoDB 聚合将集合合并到固定大小

Question

我有一个看起来像这样的集合：

{
    "_id" : id1,
    "field1" : 11,
    "field2": 101,
    "localityID" : 27
}
{
    "_id" : id2,
    "field1" : 22,
    "field2": 202,
    "localityID" : 27
}
{
    "_id" : id3,
    "field1" : 33,
    "field2": 303,
    "localityID" : 27
}
{
    "_id" : id4,
    "field1" : 44,
    "field2": 404,
    "localityID" : 27
}
{
    "_id" : id5,
    "field1" : 55,
    "field2": 505,
    "localityID" : 27
}
{
    "_id" : id6,
    "field1" : 66,
    "field2": 606,
    "localityID" : 61
}
{
    "_id" : id4,
    "field1" : 77,
    "field2": 707,
    "localityID" : 61
}

用例 - 我想以 3 的批次检索和处理具有相同 localityID 的记录。 出于跟踪目的，我还想跟踪在特定批次中处理的记录

同样，我想使用 MongoDB 的聚合框架来组合具有相同 localityID 但最大为固定大小（如上所述为 3）的集合。

我想将上面的集合更新为：

{
  "_id" : "id111",
  "batchId" : "batch1",
  "localityID": 27,
  "batches": [
     {
         "field1" : 11,
         "field2": 101
     },
     {
         "field1" : 22,
         "field2": 202
     },
     {
         "field1" : 33,
         "field2": 303
     }
  ]
}
{
  "_id" : "id222",
  "batchId" : "batch2",
  "localityID": 27,
  "batches": [
     {
         "field1" : 44,
         "field2": 404
     },
     {
         "field1" : 55,
         "field2": 505
     }
  ]
}
{
  "_id" : "id333",
  "batchId" : "batch1",
  "localityID": 61,
  "batches": [
     {
         "field1" : 66,
         "field2": 606
     },
     {
         "field1" : 77,
         "field2": 707
     }
  ]
}

我尝试了几种聚合函数的组合，如下所示，但未能获得预期的结果。

（这能够将所有具有相同localityID 的记录组合在一起但只能在一个文档中，而不是批量）

db.old_collection.aggregate([
  { "$group": { "_id": "$localityID" } },
  { "$lookup": {
    "from": "old_collection",
    "let": { "lid": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$localityID", "$$lid" ] } }},
      { "$project": { "_id": 0, "field1": 1, "field2": 1 } }
    ],
    "as": "batches"
  }},
  {"$out": "new_collection" }
])

上述聚合函数产生以下结果 -

{
  "_id" : "id111",
  "batchId" : "batch1",
  "localityID": 27,
  "batches": [
     {
         "field1" : 11,
         "field2": 101
     },
     {
         "field1" : 22,
         "field2": 202
     },
     {
         "field1" : 33,
         "field2": 303
     },
     {
         "field1" : 44,
         "field2": 404
     },
     {
         "field1" : 55,
         "field2": 505
     }
  ]
}
{
  "_id" : "id333",
  "batchId" : "batch1",
  "localityID": 61,
  "batches": [
     {
         "field1" : 66,
         "field2": 606
     },
     {
         "field1" : 77,
         "field2": 707
     }
  ]
}

这可以通过 Mongo 的聚合框架实现吗？或者我会用其他东西更好吗？

Answer 1

这个想法来自this answer。 您可以使用$range 生成一个索引数组，其中步骤参数设置为一些bucketSize。那么你只需要$slice 就可以得到一个大小为bucketSize 的数组，试试这个：

let bucketSize = 3;

db.old_collection.aggregate([
    {
        $group: {
            _id: "$localityID",
            id: { $first: "$_id" },
            localityID: { $first: "$localityID" },
            batches: {
                $push: {
                    field1: "$field1",
                    field2: "$field2"
                }
            }
        }
    },
    {
        $project: {
            _id: 0,
            localityID: "$localityID",
            batches: {
                $map: {
                    input: { $range: [0, { $size: "$batches" }, bucketSize] },
                    as: "index",
                    in: { $slice: ["$batches", "$$index", bucketSize] }
                }
            }
        }
    },
    {
        $unwind: {
            path: "$batches",
            includeArrayIndex: "batchId"
        }
    },
    {
        $addFields: {
            batchId: {
                $concat: [
                    "batch",
                    { $toString: { $add: ["$batchId", 1] } }
                ]
            }
        }
    },
    // $sort is optional. You can remove it if not required.
    {
        $sort: {
            localityID: 1,
            batchId: 1
        }
    }
    { $out: "new_collection" }
]);

输出

[
    {
        "_id": ObjectId("..."),
        "localityID": 27,
        "batches": [
            {
                "field1": 11,
                "field2": 101
            },
            {
                "field1": 22,
                "field2": 202
            },
            {
                "field1": 33,
                "field2": 303
            }
        ],
        "batchId": "batch1"
    },
    {
        "_id": ObjectId("..."),
        "localityID": 27,
        "batches": [
            {
                "field1": 44,
                "field2": 404
            },
            {
                "field1": 55,
                "field2": 505
            }
        ],
        "batchId": "batch2"
    },
    {
        "_id": ObjectId("..."),
        "localityID": 61,
        "batches": [
            {
                "field1": 66,
                "field2": 606
            },
            {
                "field1": 77,
                "field2": 707
            }
        ],
        "batchId": "batch1"
    }
]

Answer 2

如前所述，我不了解字段 batchId 的逻辑。除此之外，简单的解决方案可能是这个：

db.collection.aggregate([
   { $group: { _id: "$localityID", batches: { $push: { field1: "$field1", field2: "$field2" } } } },
   {
      $project: {
         localityID: "$_id",
         batches: { $slice: ["$batches", 1, 3] }
      }
   }
])