Nodejs REST API使用MongoDB db.collection（...）。findAsync方法在Heroku中失败，但在本地环境中工作

Question

我开发了一个Nodejs REST搜索API，用蓝鸟承诺API从mongodb返回数据。此API在本地工作，但在Heroku中失败，错误如下。

顺便说一句，我在Heroku中有其他REST API正常工作。我想这可能与env /配置有关。请详细说明一下。

TypeError: db.collection(...).findAsync is not a function
    at next (/app/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/app/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] 
    (/app/node_modules/express/lib/router/layer.js:95:5)
    at param (/app/node_modules/express/lib/router/index.js:365:14)
    at next (/app/node_modules/express/lib/router/index.js:275:10)
    at Function.handle (/app/node_modules/express/lib/router/index.js:174:3)
    at /app/node_modules/express/lib/router/index.js:281:22
    at router (/app/node_modules/express/lib/router/index.js:47:12)
    at param (/app/node_modules/express/lib/router/index.js:354:14)
    at Layer.handle [as handle_request] 
    (/app/node_modules/express/lib/router/layer.js:95:5)
    at Function.process_params 
    (/app/node_modules/express/lib/router/index.js:410:3)
    at trim_prefix (/app/node_modules/express/lib/router/index.js:317:13)
    at /app/node_modules/express/lib/router/index.js:284:7
    at Layer.handle [as handle_request] 
    (/app/node_modules/express/lib/router/layer.js:95:5)
    at Function.process_params 
    (/app/node_modules/express/lib/router/index.js:335:12)
    at next (/app/node_modules/express/lib/router/index.js:275:10)
    at trim_prefix (/app/node_modules/express/lib/router/index.js:317:13)
    at /app/node_modules/express-fileupload/lib/index.js:18:14
    at /app/node_modules/express/lib/router/index.js:284:7

code is below:


    const mongoose = require('../../../utils/mongodb');
    const Promise = require('bluebird');
    const mongoDB = require('mongodb');
    Promise.promisifyAll(mongoDB);
    const db = mongoose.connection.db;

    exports.searchChat = function (req, res) {
    const userId = req.params.userid;

    const getChatArray = function () {
    return db.collection('chats').findAsync({ participants: { 
    $elemMatch: { $eq: userId } } }).then(chatCursor => 
    chatCursor.toArrayAsync());};

    const promises = getChatArray().map(chat => 
    db.collection('messages').findAsync({ chat: { $eq: chat._id } 
    }).then(msgCursor => msgCursor.toArrayAsync()));

    Promise.all(promises).then((messages) =>{res.json(messages); }).catch(
    reason => chatlog.error(`retrieve chat error: ${reason}`));
    };