用排序和限制填充虚拟

Question

我正在尝试编写一个猫鼬查询，该查询将检索一组资产以及这些资产的最新事务。交易与资产是独立的集合。

为此，我首先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
})

然后在一个.populate快速控制器中使用node.js查询(注意硬编码的“限制: 1”在某个时候会变成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);
        });
  }

在我的测试系统中有3个资产，每一个都有一堆事务，它返回的是前两个资产的旧事务，而不是第三个资产的旧事务。

当我删除“限制:1”并返回所有事务时，它正确地排序并返回所有三个资产的事务。

我相信我会遇到这样的问题：

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

有什么好的解决办法吗？

返回所有事务将不是一个可行的长期解决方案.

Answer 1

这确实是个卷曲的问题。这里的基本知识是，.populate() (当然不是“用虚拟人填充”)的设计并不能像您在这里所期望的那样工作。

问题解释

本质上，.populate()本质上是向MongoDB发出的另一个查询，用于检索相关数据。为此，它基本上发出一个带有$in的查询，其中包含要匹配的目标的所有“相关字段”值。

“4321号”的核心是，对于诸如"sort"和"limit"这样的选项，需要提供这样一个$in参数的实际查询实际上是一个.aggregate()语句，它能够“获得每个键分组的最后一个n结果”。这实际上不是猫鼬目前向MongoDB发出的问题，也不适合在这个时候根据可用的操作对n项进行分组。

您可以手动使用.aggregate()，如所提供的清单末尾所示，当然只有在有限的情况下才能做到这一点。

  // 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
    }}
  ])

这不是一个很好的解决方案，也不是一个真正有效的解决方案，但比其他类似的解决方案更好。我相信有一种更好的方法。

更好的解决办法

对于这里的情况，我怀疑有许多类似的情况，您不希望将“完整”事务列表包含在父文档中，甚至不希望将其作为引用，因为所产生的数组的潜在大小。这种“反模式”通常是“虚拟填充”以及$lookup设计所要避免的。

但是，在“获取最新事务”的特定用例中，这两种方法都不是可行的解决方案。因为这两种方法本质上都需要查看“所有”事务，然后只从它们检索n结果。

因此，这里的“最新”或“最近”案例实际上退回到“嵌入”(至少是引用)在“有限数量”上，以便提供一个可行的解决方案。因此，我们的建议就是做到这一点，并在父程序中保留一个事务的"recent"列表。这为您提供了包含如下内容的场景中的文档：

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

这里指出，这些不是“所有”相关交易，而是“最近”的交易。重点是，只保留一个“小”的清单，以适当的目的。

通过这种方式，您可以直接查询“父”，只需将$slice从"recent"项包含的数组中查询即可。在清单中，我是这样做的：

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

它从数组中返回“最新”条目，而不需要向服务器进行任何其他查询。在本例中，它是“最新的”，因为我们在写入"transactions"集合的同时，将项“预先”添加到该数组中，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
        }
      }}
    )

其中的关键元素是父级数组中的$push，该元素被$position修改为数组开头的"preprend“，因此”第一个“项始终是要添加到父级的”最新“事务。

然后，这里使用$slice修饰符将"recent"数组保持在n项的限制范围内。因此，随着新项目的添加，“最老”项将被“推开”。

此外，这里还有另一个实际目的: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，对新的“页面”的附加请求，可以通过$nin过滤出$nin数组中包含的项，并使用常规的.skip()和.limit()或交替的“范围”分页实践来检索每个“页面”的结果。

要完整地演示所有概念，请参见下面的清单和所有生成的结果。

演示清单：

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"
      }
    ]
  }
]

Answer 2

对于那些在2021年阅读这篇文章的人，从mongoose 5.12.3开始，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({})
        .populate({
            path: 'transactions',
        })
        .lean()
        .exec(function (err, model) {
            if (err)
                res.send(err);
            res.json(model);
        });
  }