Mongoose使用排序和限制填充虚拟

时间：2017-07-03 03:01:38

标签： node.js mongodb mongoose mongodb-query

我正在尝试编写一个mongoose查询，该查询检索一组资产以及这些资产的最新事务。交易与资产分开收集。

为实现此目的，首先我在资产模型中创建了一个虚拟数组，以将资产链接到事务。 

schema.virtual('transactions', {
    ref: 'transaction',
    localField: '_id',
    foreignField: '_asset',
    justOne: false
})

然后我在node.js express控制器中使用.populate进行查询（注意硬编码的“limit：1”将在某个时刻变为N）：

exports.getList = function (req, res) {
    Model
        .find({}, { __v: 0 })
        .populate({
            path: 'transactions',
            options: { sort: { 'created_at': -1}, limit: 1},
        })
        .lean()
        .exec(function (err, model) {
            if (err)
                res.send(err);
            res.json(model);
        });
  }

我的测试系统中有3个资产，每个资产都有一堆交易，它返回前两个资产的旧交易，而第三个资产则没有。

当我删除“limit：1”并返回所有交易时，它会正确排序并返回所有三个资产的交易。

我相信我遇到了这个错误：

https://github.com/Automattic/mongoose/issues/4321

关于优雅解决方法的任何想法？
退回所有交易将不是一个可行的长期解决方案。

2 个答案:

答案 0 :(得分：5)

这确实是一个卷曲的问题。这里的基础是.populate()，当然不是“填充虚拟”，不是像你期望的那样工作。

问题解释

本质上.populate()本质上是另一个发给MongoDB以检索相关数据的查询。为此，它基本上发出一个$in的查询，其中包含要匹配的目标的所有“相关字段”值。

"issue 4321"的核心是"sort"和"limit"这样的选项，需要通过这样的$in参数提供的实际查询是事实上，.aggregate()语句可以“获取为每个键分组的最后n个结果”。这实际上并不是mongoose目前向MongoDB发布的内容，此时通过可用操作对n项进行分组也不实际。

您可以手动使用.aggregate()来解决这个问题，如提供的列表末尾所示，但当然只有少数几种情况。

  // Get latest transactions for each master
  Transaction.aggregate([
    { '$match': {
      '_asset': {
        '$in': masters.map(m => m._id)
      }
    }},
    { '$sort': { '_asset': 1, 'createdAt': -1 } },
    { '$group': {
      '_id': '$_asset',
      'amount': { '$first': '$amount' },
      'createdAt': { '$first': '$createdAt' },
      'updatedAt': { '$first': '$updatedAt' },
      'did': { '$first': '$_id' }
    }},
    { '$project': {
      '_id': '$did',
      '_asset': '$_id',
      'amount': 1,
      'createdAt': 1,
      'updatedAt': 1
    }}
  ])

这不是很好，也不是一个真正高效的解决方案，但比其他类似的替代方案更好。我相信这是一种更好的方式。

更好的解决方案

对于你的情况，我怀疑很多类似的情况，你不希望在父文档中包含“完整”事务列表，即使作为引用，因为生成的数组的潜在大小。这种“反模式”通常是“虚拟填充”，实际上是$lookup旨在避免的。

在“获取最新交易”的特定用例中，这些都不是可行的解决方案。因为两者都基本上需要查看“所有”事务，然后只从它们检索n结果。

因此，这里的“最新”或实际上“最近”的案例实际上回归到“嵌入”（至少是参考）“有限数量”，以便提供可行的解决方案。因此，建议就是这样做，并在父本身内保留"recent"个事务列表。这为您提供了包含的场景中的文档，如下所示：

{
        "_id" : ObjectId("5959e34adf833e1451a32661"),
        "__v" : 0,
        "name" : "One",
        "recent" : [
                ObjectId("5959e34bdf833e1451a32676"),
                ObjectId("5959e34bdf833e1451a32674"),
                ObjectId("5959e34bdf833e1451a32672"),
                ObjectId("5959e34bdf833e1451a32670"),
                ObjectId("5959e34bdf833e1451a3266e")
        ]
}

在此注意到这些并非“全部”相关交易，而只是“最新”交易。关键是只保留一个适合目的的“小”列表。

通过这种方式，您可以直接查询“父”，并只查询"recent"项所包含的数组$slice。在列表中我这样做：

Master.find().select({ 'recent': { '$slice': 1 } })

返回数组中的“latest”条目，而不向服务器发送任何其他查询。在这种情况下，它是“最新的”，因为我们在写入"transactions"集合的同时将“prepend”项添加到此数组中，该集合包含所有内容：

Transaction.create({ _asset: master._id, amount: data.amount })
  .then(transaction =>
    Master.update(
      { _id: transaction._asset },
      { "$push": {
        "recent": {
          "$each": [transaction._id],
          "$position": 0,
          "$slice": 5
        }
      }}
    )

关键元素是父项中数组的$push，它在数组的开头用$position修改为“预先读取”，因此“第一个”项始终是“最新的“与父母相关的交易。

然后，此处使用$slice修饰符将"recent"数组仅限制为n项的限制。因此，当添加较新的项目时，“最旧的”项目将被“推离”列表。

此外还有另一个实际目的，即在分页场景中列出“主”和“事务”时，第一个请求可以直接使用"recent"数组。然后，对新“网页”的其他请求只需通过$nin过滤出"recent"数组中包含的项目，并使用常规.skip()和.limit()或替代“范围”分页练习，以检索结果的每个“页面”。

要完整演示所有概念，请参阅下面的列表和所有生成的结果。

演示列表： 

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

mongoose.set('debug',true);
mongoose.Promise = global.Promise;

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

const transactionSchema = new Schema({
  _asset: { type: Schema.Types.ObjectId, ref: 'Master' },
  amount: Number
},{
  timestamps: {
    createdAt: 'createdAt'
  }
});


const Transaction = mongoose.model('Transaction', transactionSchema);

const masterSchema = new Schema({
  name: String,
  recent: [{ type: Schema.Types.ObjectId, ref: 'Transaction' }]
});

masterSchema.virtual('transactions', {
  ref: 'Transaction',
  localField: '_id',
  foreignField: '_asset',
  justOne: false
});


const Master = mongoose.model('Master', masterSchema);

function log(data) {
  console.log(JSON.stringify(data, undefined, 2))
}

async.series(
  [
    // Clean data
    (callback) =>
      async.each(mongoose.models,(model,callback) =>
        model.remove({},callback),callback),

    // Create Masters
    (callback) =>
      Master.insertMany(['One','Two'].map( name => ({ name })),callback),

    // Add 10 transactions to each master
    (callback) =>
      async.each(['One','Two'],(name,callback) =>
        async.eachSeries(
          Array.apply(null,Array(10)).map((e,i) => ({ name, amount: i+1 })),
          (data,callback) => {
            Master.findOne({ name: data.name })
              .then(master =>
                Transaction.create({ _asset: master._id, amount: data.amount })
              )
              .then(transaction =>
                Master.update(
                  { _id: transaction._asset },
                  { "$push": {
                    "recent": {
                      "$each": [transaction._id],
                      "$position": 0,
                      "$slice": 5
                    }
                  }}
                )
              )
              .then(res => callback())
              .catch(callback)
          },
          callback
        ),
      callback),

    // Show populated recent 1 entry only
    (callback) =>
      Master.find().select({ 'recent': { '$slice': 1 } })
        .populate('recent').exec((err,results) => {
        if (err) callback(err);
        log(results);
        callback();
      }),

    // Populate recent - page 1 then fetch next page
    (callback) =>
      async.waterfall(
        [
          (callback) =>
            Master.findOne({ name: 'One' }).populate('recent')
              .lean()
              .exec((err,master) => {
                if (err) callback(err);
                log(master);
                callback(null,{
                  _asset: master._id,
                  exclude: master.recent.map( r => r._id )
                });
              }),

          (options,callback) =>
            Transaction.find({
              _asset: options._asset,
              _id: { '$nin': options.exclude }
            }).sort({ 'createdAt': -1 }).limit(5)
            .exec((err,transactions) => {
              if (err) callback(err);
              log(transactions)
              callback();
            })

        ],
        callback
      ),

    // Issue 4321 - Fix - Manual populate with aggregate
    (callback) =>
      Master.find().select('-recent').exec()
        .then(masters => {
          // Get latest transactions for each master
          Transaction.aggregate([
            { '$match': {
              '_asset': {
                '$in': masters.map(m => m._id)
              }
            }},
            { '$sort': { '_asset': 1, 'createdAt': -1 } },
            { '$group': {
              '_id': '$_asset',
              'amount': { '$first': '$amount' },
              'createdAt': { '$first': '$createdAt' },
              'updatedAt': { '$first': '$updatedAt' },
              'did': { '$first': '$_id' }
            }},
            { '$project': {
              '_id': '$did',
              '_asset': '$_id',
              'amount': 1,
              'createdAt': 1,
              'updatedAt': 1
            }}
          ]).exec((err,transactions) => {
            // Map latest transactions to master
            masters = masters.map(
              m => Object.assign(
                m.toObject(),
                {
                  transactions: transactions.filter(
                    t => t._asset.toHexString() === m._id.toHexString()
                  )
                }
              )
            );

            log(masters);
            callback();
          })
        }).catch(callback)

  ],
  (err) => {
    if (err) throw err;
    mongoose.disconnect();
  }
);

演示输出 

Mongoose: transactions.remove({}, {})
Mongoose: masters.remove({}, {})
Mongoose: masters.insertMany([ { __v: 0, name: 'One', _id: 5959e34adf833e1451a32661, recent: [] }, { __v: 0, name: 'Two', _id: 5959e34adf833e1451a32662, recent: [] } ], null)
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:14 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:14 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 1, _id: ObjectId("5959e34adf833e1451a32663"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 1, _id: ObjectId("5959e34adf833e1451a32664"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34adf833e1451a32664") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34adf833e1451a32663") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 2, _id: ObjectId("5959e34bdf833e1451a32665"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 2, _id: ObjectId("5959e34bdf833e1451a32666"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32666") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32665") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 3, _id: ObjectId("5959e34bdf833e1451a32667"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 3, _id: ObjectId("5959e34bdf833e1451a32668"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32668") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32667") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 4, _id: ObjectId("5959e34bdf833e1451a32669"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 4, _id: ObjectId("5959e34bdf833e1451a3266a"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266a") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32669") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 5, _id: ObjectId("5959e34bdf833e1451a3266b"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 5, _id: ObjectId("5959e34bdf833e1451a3266c"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266c") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266b") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 6, _id: ObjectId("5959e34bdf833e1451a3266d"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 6, _id: ObjectId("5959e34bdf833e1451a3266e"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266e") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266d") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 7, _id: ObjectId("5959e34bdf833e1451a3266f"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 7, _id: ObjectId("5959e34bdf833e1451a32670"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32670") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a3266f") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 8, _id: ObjectId("5959e34bdf833e1451a32671"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 8, _id: ObjectId("5959e34bdf833e1451a32672"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32672") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32671") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 9, _id: ObjectId("5959e34bdf833e1451a32673"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 9, _id: ObjectId("5959e34bdf833e1451a32674"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32674") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32673") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: masters.findOne({ name: 'Two' }, { fields: {} })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32662"), amount: 10, _id: ObjectId("5959e34bdf833e1451a32675"), __v: 0 })
Mongoose: transactions.insert({ updatedAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), createdAt: new Date("Mon, 03 Jul 2017 06:25:15 GMT"), _asset: ObjectId("5959e34adf833e1451a32661"), amount: 10, _id: ObjectId("5959e34bdf833e1451a32676"), __v: 0 })
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32661") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32676") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.update({ _id: ObjectId("5959e34adf833e1451a32662") }, { '$push': { recent: { '$each': [ ObjectId("5959e34bdf833e1451a32675") ], '$slice': 5, '$position': 0 } } }, {})
Mongoose: masters.find({}, { fields: { recent: { '$slice': 1 } } })
Mongoose: transactions.find({ _id: { '$in': [ ObjectId("5959e34bdf833e1451a32676"), ObjectId("5959e34bdf833e1451a32675") ] } }, { fields: {} })
[
  {
    "_id": "5959e34adf833e1451a32661",
    "__v": 0,
    "name": "One",
    "recent": [
      {
        "_id": "5959e34bdf833e1451a32676",
        "updatedAt": "2017-07-03T06:25:15.282Z",
        "createdAt": "2017-07-03T06:25:15.282Z",
        "_asset": "5959e34adf833e1451a32661",
        "amount": 10,
        "__v": 0
      }
    ]
  },
  {
    "_id": "5959e34adf833e1451a32662",
    "__v": 0,
    "name": "Two",
    "recent": [
      {
        "_id": "5959e34bdf833e1451a32675",
        "updatedAt": "2017-07-03T06:25:15.280Z",
        "createdAt": "2017-07-03T06:25:15.280Z",
        "_asset": "5959e34adf833e1451a32662",
        "amount": 10,
        "__v": 0
      }
    ]
  }
]
Mongoose: masters.findOne({ name: 'One' }, { fields: {} })
Mongoose: transactions.find({ _id: { '$in': [ ObjectId("5959e34bdf833e1451a32676"), ObjectId("5959e34bdf833e1451a32674"), ObjectId("5959e34bdf833e1451a32672"), ObjectId("5959e34bdf833e1451a32670"), ObjectId("5959e34bdf833e1451a3266e") ] } }, { fields: {} })
{
  "_id": "5959e34adf833e1451a32661",
  "__v": 0,
  "name": "One",
  "recent": [
    {
      "_id": "5959e34bdf833e1451a32676",
      "updatedAt": "2017-07-03T06:25:15.282Z",
      "createdAt": "2017-07-03T06:25:15.282Z",
      "_asset": "5959e34adf833e1451a32661",
      "amount": 10,
      "__v": 0
    },
    {
      "_id": "5959e34bdf833e1451a32674",
      "updatedAt": "2017-07-03T06:25:15.264Z",
      "createdAt": "2017-07-03T06:25:15.264Z",
      "_asset": "5959e34adf833e1451a32661",
      "amount": 9,
      "__v": 0
    },
    {
      "_id": "5959e34bdf833e1451a32672",
      "updatedAt": "2017-07-03T06:25:15.216Z",
      "createdAt": "2017-07-03T06:25:15.216Z",
      "_asset": "5959e34adf833e1451a32661",
      "amount": 8,
      "__v": 0
    },
    {
      "_id": "5959e34bdf833e1451a32670",
      "updatedAt": "2017-07-03T06:25:15.195Z",
      "createdAt": "2017-07-03T06:25:15.195Z",
      "_asset": "5959e34adf833e1451a32661",
      "amount": 7,
      "__v": 0
    },
    {
      "_id": "5959e34bdf833e1451a3266e",
      "updatedAt": "2017-07-03T06:25:15.180Z",
      "createdAt": "2017-07-03T06:25:15.180Z",
      "_asset": "5959e34adf833e1451a32661",
      "amount": 6,
      "__v": 0
    }
  ]
}
Mongoose: transactions.find({ _id: { '$nin': [ ObjectId("5959e34bdf833e1451a32676"), ObjectId("5959e34bdf833e1451a32674"), ObjectId("5959e34bdf833e1451a32672"), ObjectId("5959e34bdf833e1451a32670"), ObjectId("5959e34bdf833e1451a3266e") ] }, _asset: ObjectId("5959e34adf833e1451a32661") }, { sort: { createdAt: -1 }, limit: 5, fields: {} })
[
  {
    "_id": "5959e34bdf833e1451a3266c",
    "updatedAt": "2017-07-03T06:25:15.164Z",
    "createdAt": "2017-07-03T06:25:15.164Z",
    "_asset": "5959e34adf833e1451a32661",
    "amount": 5,
    "__v": 0
  },
  {
    "_id": "5959e34bdf833e1451a3266a",
    "updatedAt": "2017-07-03T06:25:15.135Z",
    "createdAt": "2017-07-03T06:25:15.135Z",
    "_asset": "5959e34adf833e1451a32661",
    "amount": 4,
    "__v": 0
  },
  {
    "_id": "5959e34bdf833e1451a32668",
    "updatedAt": "2017-07-03T06:25:15.080Z",
    "createdAt": "2017-07-03T06:25:15.080Z",
    "_asset": "5959e34adf833e1451a32661",
    "amount": 3,
    "__v": 0
  },
  {
    "_id": "5959e34bdf833e1451a32666",
    "updatedAt": "2017-07-03T06:25:15.039Z",
    "createdAt": "2017-07-03T06:25:15.039Z",
    "_asset": "5959e34adf833e1451a32661",
    "amount": 2,
    "__v": 0
  },
  {
    "_id": "5959e34adf833e1451a32664",
    "updatedAt": "2017-07-03T06:25:15.009Z",
    "createdAt": "2017-07-03T06:25:15.009Z",
    "_asset": "5959e34adf833e1451a32661",
    "amount": 1,
    "__v": 0
  }
]
Mongoose: masters.find({}, { fields: { recent: 0 } })
Mongoose: transactions.aggregate([ { '$match': { _asset: { '$in': [ 5959e34adf833e1451a32661, 5959e34adf833e1451a32662 ] } } }, { '$sort': { _asset: 1, createdAt: -1 } }, { '$group': { _id: '$_asset', amount: { '$first': '$amount' }, createdAt: { '$first': '$createdAt' }, updatedAt: { '$first': '$updatedAt' }, did: { '$first': '$_id' } } }, { '$project': { _id: '$did', _asset: '$_id', amount: 1, createdAt: 1, updatedAt: 1 } } ], {})
[
  {
    "_id": "5959e34adf833e1451a32661",
    "__v": 0,
    "name": "One",
    "transactions": [
      {
        "amount": 10,
        "createdAt": "2017-07-03T06:25:15.282Z",
        "updatedAt": "2017-07-03T06:25:15.282Z",
        "_id": "5959e34bdf833e1451a32676",
        "_asset": "5959e34adf833e1451a32661"
      }
    ]
  },
  {
    "_id": "5959e34adf833e1451a32662",
    "__v": 0,
    "name": "Two",
    "transactions": [
      {
        "amount": 10,
        "createdAt": "2017-07-03T06:25:15.280Z",
        "updatedAt": "2017-07-03T06:25:15.280Z",
        "_id": "5959e34bdf833e1451a32675",
        "_asset": "5959e34adf833e1451a32662"
      }
    ]
  }
]

答案 1 :(得分：3)

对于在 2021 年阅读本文的人，从 mongoose 5.12.3 起，可以在模型中完成以下操作：

schema.virtual('transactions', {
    ref: 'transaction',
    localField: '_id',
    foreignField: '_asset',
    justOne: false,
    options: { sort: { 'createdAt': -1}, limit: 1},
})

然后在控制器中：

exports.getList = function (req, res) {
    Model
        .find({}, { __v: 0 })
        .populate({
            path: 'transactions',
        })
        .lean()
        .exec(function (err, model) {
            if (err)
                res.send(err);
            res.json(model);
        });
  }
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