我想合并两个聚合操作。第一个操作返回，例如:

{ "_id" : "Colors", "count" : 12 }
{ "_id" : "Animals", "count" : 6 }

第二个操作返回，例如:

{ "_id" : "Red", "count" : 10 }
{ "_id" : "Blue", "count" : 9 }
{ "_id" : "Green", "count" : 9 }
{ "_id" : "White", "count" : 7 }
{ "_id" : "Yellow", "count" : 7 }
{ "_id" : "Orange", "count" : 7 }
{ "_id" : "Black", "count" : 5 }
{ "_id" : "Goose", "count" : 4 }
{ "_id" : "Chicken", "count" : 3 }
{ "_id" : "Grey", "count" : 3 }
{ "_id" : "Cat", "count" : 3 }
{ "_id" : "Rabbit", "count" : 3 }
{ "_id" : "Duck", "count" : 3 }
{ "_id" : "Turkey", "count" : 2 }
{ "_id" : "Elephant", "count" : 2 }
{ "_id" : "Shark", "count" : 2 }
{ "_id" : "Fish", "count" : 2 }
{ "_id" : "Tiger", "count" : 2 }
{ "_id" : "Purple", "count" : 1 }
{ "_id" : "Pink", "count" : 1 }

如何结合这 2 个操作来实现以下目标？

{ "_id" : "Colors", "count" : 12, "items" :
    [
        { "_id" : "Red", "count" : 10 },
        { "_id" : "Blue", "count" : 9 },
        { "_id" : "Green", "count" : 9 },
        { "_id" : "White", "count" : 7 },
        { "_id" : "Yellow", "count" : 7 },
        { "_id" : "Orange", "count" : 7 },
        { "_id" : "Black", "count" : 5 },
        { "_id" : "Grey", "count" : 3 },
        { "_id" : "Purple", "count" : 1 },
        { "_id" : "Pink", "count" : 1 }
    ]
},
{ "_id" : "Animals", "count" : 6, "items" :
    [
        { "_id" : "Goose", "count" : 4 },
        { "_id" : "Chicken", "count" : 3 },
        { "_id" : "Cat", "count" : 3 },
        { "_id" : "Rabbit", "count" : 3 },
        { "_id" : "Duck", "count" : 3 },
        { "_id" : "Turkey", "count" : 2 },
        { "_id" : "Elephant", "count" : 2 },
        { "_id" : "Shark", "count" : 2 },
        { "_id" : "Fish", "count" : 2 },
        { "_id" : "Tiger", "count" : 2 }
    ]
}

架构

var ListSchema = new Schema({
    created: {
        type: Date,
        default: Date.now
    },
    title: {
        type: String,
        default: '',
        trim: true,
        required: 'Title cannot be blank'
    },
    items: {
        type: Array,
        default: [String],
        trim: true
    },
    creator: {
        type: Schema.ObjectId,
        ref: 'User'
    }
});

操作一

db.lists.aggregate(
      [
        { $group: { _id: "$title", count: { $sum: 1 } } },
        { $sort: { count: -1 } }
      ]
    )

操作2

db.lists.aggregate(
      [
        { $unwind: "$items" },
        { $group: { _id: "$items", count: { $sum: 1 } } },
        { $sort: { count: -1 } }
      ]
    )

最佳答案

这实际上取决于您在响应中所追求的结果类型。您询问的内容似乎表明您正在寻找结果中的“方面计数”，但稍后我会谈到这一点。

作为基本结果，这种方法没有任何问题:

    Thing.aggregate(
      [
        { "$group": {
          "_id": {
            "type": "$type", "name": "$name"
          },
          "count": { "$sum": 1 }
        }},
        { "$group": {
          "_id": "$_id.type",
          "count": { "$sum": "$count" },
          "names": {
            "$push": { "name": "$_id.name", "count": "$count" }
          }
        }}
      ],
      function(err,results) {
        console.log(JSON.stringify(results, undefined, 2));
        callback(err);
      }
    )

这应该会给你这样的结果:

[
  {
    "_id": "colours",
    "count": 50102,
    "names": [
      { "name": "Green",  "count": 9906  },
      { "name": "Yellow", "count": 10093 },
      { "name": "Red",    "count": 10083 },
      { "name": "Orange", "count": 9997  },
      { "name": "Blue",   "count": 10023 }
    ]
  },
  {
    "_id": "animals",
    "count": 49898,
    "names": [
      { "name": "Tiger",    "count": 9710  },
      { "name": "Lion",     "count": 10058 },
      { "name": "Elephant", "count": 10069 },
      { "name": "Monkey",   "count": 9963  },
      { "name": "Bear",     "count": 10098 }
    ]
  }
]

这里最基本的方法是简单地 $group分两个阶段，第一阶段将键组合聚合到最低(最细粒度)分组级别，然后再次处理 $group 以基本上“加起来”最高级别的总数(最小粒度)分组级别，因此也将较低的结果添加到项目数组中。

但这并不像在“方面计数”中那样“分离”，因此这样做变得有点复杂，也有点疯狂。但首先是示例:

    Thing.aggregate(
      [
        { "$group": {
          "_id": {
            "type": "$type",
            "name": "$name"
          },
          "count": { "$sum": 1 }
        }},
        { "$group": {
          "_id": "$_id.type",
          "count": { "$sum": "$count" },
          "names": {
            "$push": { "name": "$_id.name", "count": "$count" }
          }
        }},
        { "$group": {
          "_id": null,
          "types": {
            "$push": {
              "type": "$_id", "count": "$count"
            }
          },
          "names": { "$push": "$names" }
        }},
        { "$unwind": "$names" },
        { "$unwind": "$names" },
        { "$group": {
          "_id": "$types",
          "names": { "$push": "$names" }
        }},
        { "$project": {
          "_id": 0,
          "facets": {
            "types": "$_id",
            "names": "$names",
          },
          "data": { "$literal": [] }
        }}
      ],
      function(err,results) {
        console.log(JSON.stringify(results[0], undefined, 2));
        callback(err);
      }
    );

这将产生这样的输出:

{
  "facets": {
    "types": [
      { "type": "colours", "count": 50102 },
      { "type": "animals", "count": 49898 }
    ],
    "names": [
      { "name": "Green",    "count": 9906  },
      { "name": "Yellow",   "count": 10093 },
      { "name": "Red",      "count": 10083 },
      { "name": "Orange",   "count": 9997  },
      { "name": "Blue",     "count": 10023 },
      { "name": "Tiger",    "count": 9710  },
      { "name": "Lion",     "count": 10058 },
      { "name": "Elephant", "count": 10069 },
      { "name": "Monkey",   "count": 9963  },
      { "name": "Bear",     "count": 10098 }
    ]
  },
  "data": []
}

但显而易见的是，虽然“可能”，但在管道中进行的这种“杂耍”以产生这种输出格式并不是真正有效的。与第一个示例相比，这里有很多开销只是简单地将结果拆分为它们自己的数组响应并且独立于分组键。随着要生成的“方面”越来越多，这明显变得更加复杂。

此外，正如此处输出中所暗示的，人们通常对“方面计数”的要求是，除了聚合方面之外，结果“数据”也包含在响应(可能已分页)中。因此，进一步的并发症应该在这里显而易见:

        { "$group": {
          "_id": null,
          (...)

此类操作的要求基本上是将每条数据“填充”到单个对象中。在大多数情况下，当然在您想要结果中的实际数据(在此示例中使用 100,000)的情况下，遵循这种方法变得完全不切实际，并且几乎肯定会超过 16MB 的 BSON 文档大小限制。

在这种情况下，如果您想在响应中生成结果和该数据的“方面”，那么这里最好的方法是将每个聚合和输出页面作为单独的查询操作运行，并“流式传输”输出JSON(或其他格式)返回给接收客户端。

作为一个独立的例子:

var async = require('async'),
    mongoose = require('mongoose'),
    Schema = mongoose.Schema;


mongoose.connect('mongodb://localhost/things');

var data = {
      "colours": [
        "Red","Blue","Green","Yellow","Orange"
      ],
      "animals": [
        "Lion","Tiger","Bear","Elephant","Monkey"
      ]
    },
    dataKeys = Object.keys(data);

var thingSchema = new Schema({
  "name": String,
  "type": String
});

var Thing = mongoose.model( 'Thing', thingSchema );

var writer = process.stdout;

mongoose.connection.on("open",function(err) {
  if (err) throw err;
  async.series(
    [
      function(callback) {
        process.stderr.write("removing\n");
        Thing.remove({},callback);
      },
      function(callback) {
        process.stderr.write("inserting\n");
        var bulk = Thing.collection.initializeUnorderedBulkOp(),
            count = 0;

        async.whilst(
          function() { return count < 100000; },
          function(callback) {
            var keyLen    = dataKeys.length,
                keyIndex  = Math.floor(Math.random(keyLen)*keyLen),
                type      = dataKeys[keyIndex],
                types     = data[type],
                typeLen   = types.length,
                nameIndex = Math.floor(Math.random(typeLen)*typeLen),
                name      = types[nameIndex];

            var obj = { "type": type, "name": name };
            bulk.insert(obj);
            count++;

            if ( count % 1000 == 0 ) {
              process.stderr.write('insert count: ' + count + "\n");
              bulk.execute(function(err,resp) {
                bulk = Thing.collection.initializeUnorderedBulkOp();
                callback(err);
              });
            } else {
              callback();
            }

          },
          callback
        );
      },

      function(callback) {
        writer.write("{ \n  \"page\": 1,\n  \"pageSize\": 25,\n")
        writer.write("  \"facets\":  {\n");      // open object response

        var stream = Thing.collection.aggregate(
          [
            { "$group": {
              "_id": "$name",
              "count": { "$sum": 1 }
            }}
          ],
          {
            "cursor": {
              "batchSize": 1000
            }
          }
        );

        var counter = 0;

        stream.on("data",function(data) {
          stream.pause();

          if ( counter == 0 ) {
            writer.write("    \"names\": [\n");
          } else {
            writer.write(",\n");
          }

          data = { "name": data._id, "count": data.count };

          writer.write("      " + JSON.stringify(data));

          counter++;
          stream.resume();
        });

        stream.on("end",function() {
          writer.write("\n    ],\n");

          var stream = Thing.collection.aggregate(
            [
              { "$group": {
                "_id": "$type",
                "count": { "$sum": 1 }
              }}
            ],
            {
              "cursor": {
                "batchSize": 1000
              }
            }
          );

          var counter = 0;
          stream.on("data",function(data) {
            stream.pause();

            if ( counter == 0 ) {
              writer.write("    \"types\": [\n");
            } else {
              writer.write(",\n");
            }

            data = { "name": data._id, "count": data.count };

            writer.write("      " + JSON.stringify(data));

            counter++;
            stream.resume();
          });

          stream.on("end",function() {
            writer.write("\n    ]\n  },\n");

            var stream = Thing.find({}).limit(25).stream();
            var counter = 0;

            stream.on("data",function(data) {
              stream.pause();
              if ( counter == 0 ) {
                writer.write("  \"data\": [\n");
              } else {
                writer.write(",\n");
              }

              writer.write("    " + JSON.stringify(data));

              counter++;
              stream.resume();

            });

            stream.on("end",function() {
                writer.write("\n  ]\n}\n");
                callback();
            });

          });

        });
      }
    ],
    function(err) {
      if (err) throw err;
      process.exit();
    }
  );
});

输出如下:

{
  "page": 1,
  "pageSize": 25,
  "facets":  {
    "names": [
      {"name":"Red","count":10007},
      {"name":"Tiger","count":10012},
      {"name":"Yellow","count":10119},
      {"name":"Monkey","count":9970},
      {"name":"Elephant","count":10046},
      {"name":"Bear","count":10082},
      {"name":"Orange","count":9982},
      {"name":"Green","count":10005},
      {"name":"Blue","count":9884},
      {"name":"Lion","count":9893}
    ],
    "types": [
      {"name":"colours","count":49997},
      {"name":"animals","count":50003}
    ]
  },
  "data": [
    {"_id":"55bf141f3edc150b6abdcc02","type":"animals","name":"Lion"},
    {"_id":"55bf141f3edc150b6abdc81b","type":"colours","name":"Blue"},
    {"_id":"55bf141f3edc150b6abdc81c","type":"colours","name":"Orange"},
    {"_id":"55bf141f3edc150b6abdc81d","type":"animals","name":"Bear"},
    {"_id":"55bf141f3edc150b6abdc81e","type":"animals","name":"Elephant"},
    {"_id":"55bf141f3edc150b6abdc81f","type":"colours","name":"Orange"},
    {"_id":"55bf141f3edc150b6abdc820","type":"colours","name":"Green"},
    {"_id":"55bf141f3edc150b6abdc821","type":"animals","name":"Lion"},
    {"_id":"55bf141f3edc150b6abdc822","type":"animals","name":"Monkey"},
    {"_id":"55bf141f3edc150b6abdc823","type":"colours","name":"Yellow"},
    {"_id":"55bf141f3edc150b6abdc824","type":"colours","name":"Yellow"},
    {"_id":"55bf141f3edc150b6abdc825","type":"colours","name":"Orange"},
    {"_id":"55bf141f3edc150b6abdc826","type":"animals","name":"Monkey"},
    {"_id":"55bf141f3edc150b6abdc827","type":"colours","name":"Blue"},
    {"_id":"55bf141f3edc150b6abdc828","type":"animals","name":"Tiger"},
    {"_id":"55bf141f3edc150b6abdc829","type":"colours","name":"Red"},
    {"_id":"55bf141f3edc150b6abdc82a","type":"animals","name":"Monkey"},
    {"_id":"55bf141f3edc150b6abdc82b","type":"animals","name":"Elephant"},
    {"_id":"55bf141f3edc150b6abdc82c","type":"animals","name":"Tiger"},
    {"_id":"55bf141f3edc150b6abdc82d","type":"animals","name":"Bear"},
    {"_id":"55bf141f3edc150b6abdc82e","type":"colours","name":"Yellow"},
    {"_id":"55bf141f3edc150b6abdc82f","type":"animals","name":"Lion"},
    {"_id":"55bf141f3edc150b6abdc830","type":"animals","name":"Elephant"},
    {"_id":"55bf141f3edc150b6abdc831","type":"colours","name":"Orange"},
    {"_id":"55bf141f3edc150b6abdc832","type":"animals","name":"Elephant"}
  ]
}

这里有一些注意事项，特别是 Mongoose .aggregate()并不真正直接支持标准节点流接口(interface)。 .cursor() 提供了一个 .each() 方法在聚合方法上，但是 core API method 隐含的“流”这里提供了更多的控制权，所以 .collection 方法在这里获取底层 driver object是可取的。希望 future 的 Mongoose 版本会考虑这一点。

因此，如果您的最终目标是这样一个“方面计数”以及此处演示的结果，那么每个聚合和结果都最适合以演示的方式“流式传输”。如果没有它，聚合就会变得过于复杂，并且很可能会超过 BSON 限制，就像在这种情况下这样做一样。

关于mongodb - 在单个结果中组合聚合操作，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31777590/