Mongodb mapReduce给Mongoerror“值是空”

Question

我试图了解mapReduce函数，并根据stackoverflow上的此问题 ，使用它按与提供的ID数组相同的顺序对响应进行排序。 当我尝试通过提供书名数组来提取书籍时，一切工作正常，但是使用id数组则行不通。 我尝试将id转换为ObjectId，但仍然没有希望。 以下是书籍的数据模型。

 const mongoose = require('mongoose'); const BookSchema = mongoose.Schema({ title: { type: 'string', required: true }, author: { type: 'string' }, coverUrl: { type: 'string', required: true }, owner: { type: 'string', required: true }, description: { type: 'string' } }); let Book = module.exports = mongoose.model('Book', BookSchema); 

提取数据的代码是

 // booksIds is an array with only two ids which are strings. // three arguments passed to callback (err, docs, stats) Book.getBooksByIdsWithOrder = function (booksIds, callback) { // let book_titles = ["The History of Street Art", "A Night In The Woods"]; booksIds[0] = mongoose.Types.ObjectId(booksIds[0]); booksIds[1] = mongoose.Types.ObjectId(booksIds[1]); let ask = {} ask.map = function () { var order = inputs.indexOf(this._id); emit( order, { doc: this } ); }; ask.out = {"inline": 1}; ask.query = { "_id": {"$in": booksIds}}; ask.scope = {"inputs": booksIds}; ask.finalize = function (key, value) { return value.doc; } Book.mapReduce(ask, callback); }; 

错误！

 { MongoError: TypeError: value is null : @:2:5 at Function.MongoError.create (/home/user/web_development/bookTradingClub/backend/node_modules/mongodb-core/lib/error.js:31:11) at /home/user/web_development/bookTradingClub/backend/node_modules/mongodb-core/lib/connection/pool.js:497:72 at authenticateStragglers (/home/user/web_development/bookTradingClub/backend/node_modules/mongodb-core/lib/connection/pool.js:443:16) at Connection.messageHandler (/home/user/web_development/bookTradingClub/backend/node_modules/mongodb-core/lib/connection/pool.js:477:5) at Socket.<anonymous> (/home/user/web_development/bookTradingClub/backend/node_modules/mongodb-core/lib/connection/connection.js:331:22) at emitOne (events.js:96:13) at Socket.emit (events.js:188:7) at readableAddChunk (_stream_readable.js:176:18) at Socket.Readable.push (_stream_readable.js:134:10) at TCP.onread (net.js:547:20) name: 'MongoError', message: 'TypeError: value is null :\\n@:2:5\\n', ok: 0, errmsg: 'TypeError: value is null :\\n@:2:5\\n', code: 139, codeName: 'JSInterpreterFailure' }