Mongodb mapReduce给出Mongoerror“Value is null”

Question

我正在尝试理解mapReduce函数，并根据此question on stackoverflow使用它按与提供的ID数组相同的顺序对响应进行排序。当我尝试通过提供一系列书籍来提取书籍时，一切正常，但使用ID数组它不起作用。我尝试将id转换为ObjectIds，但仍然没有希望。以下是书籍的数据模型。

    from random import randint 
    min = 1
    max = 6
    roll=input ('care to roll?')
    while roll == ('yes' or 'Yes'):
     print ("rolling...")
     print randint(min,max)
     print randint(min,max)
     while randint (1):
      break
    else:
      continue
    if roll == ('no' or 'No'):
     print ("fine then, see if I care")        

提取数据的代码是，

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);

错误！

// 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);
};