快递承诺链错误

Question

I've written a simple express.js server that handles REST API requests and fetches data from a MongoDB database. When I make a GET request to a specific endpoint ("localhost:8081/api/getUserData"), the promise chain doesn't work the way I want it to, and I still don't understand.

This is the error I get: "[TypeError: Cannot read property 'db' of undefined]"

var MongoClient = require('mongodb').MongoClient;
var express = require('express');
var app = express();
var rp = require("request-promise");
var cors = require('cors');

// use it before all route definitions
app.use(cors({ origin: '*' }));

/********************** REST API FUNCTIONS **********************/
app.get('/api/getUserData', function (req, res, next) {
  var context = {};
  console.log("in api getUserData")
  context.db_url = 'mongodb://localhost:27017/test';
  openDatabaseConnection(context)
    .then(getAllUserLocations)
    .then(closeDatabaseConnection)
    .then(function (context) {
      res.send(context.userLocations)
    })
    .catch(function (error) {
      console.log("ERROR :");
      console.log(error);
    })
})
/********************** END REST API FUNCTIONS **********************/

function getAllUserLocations(context) {
  context.db.collection("test").find().toArray().then(function (err, result) {
    console.log("Received from db: " + result.length + " objects");
    context.userLocations = result;
    return context;
  });
}

function openDatabaseConnection(context) {
  console.log("Opening DB connection...");
  return MongoClient.connect(context.db_url)
    .then(function (db) {
      console.log("DB connection opened.");
      context.db = db;
      return context;
    })
}

function closeDatabaseConnection(context) {
  console.log("Closing DB connection");
  return context.db.close()
    .then(function () {
      console.log("DB connection closed");
      return context;
    })
}

/********************** STARTING SERVER **********************/
var server = app.listen(8081, function () {
  var host = server.address().address
  var port = server.address().port
  console.log("Githex server listening at http://%s:%s", host, port)
})

Any help would be appreciated, and even more with an explanation because I don't understand what I've done wrong.

Thanks!

Answer 1

Just like @adeneo mentioned on the first comment, you are missing the db property. Look at your first function:

app.get('/api/getUserData', function (req, res, next) {
  var context = {};
  console.log("in api getUserData")
  context.db_url = 'mongodb://localhost:27017/test';
  openDatabaseConnection(context)
    .then(getAllUserLocations)
    .then(closeDatabaseConnection)
    .then(function (context) {
      res.send(context.userLocations)
    })
    .catch(function (error) {
      console.log("ERROR :");
      console.log(error);
    })
});

Now going through the lines within this function:

1. You setup context as an empty object

2. You added a db_url property onto the object, so far you have

    context = {db_url: "mongodb://localhost:27017/test} 

3. You pass the context object into the openDatabaseConnection function
4. Within the openDatabaseConnection function, you return a context object. This new returned object doesn't get set anywhere, it just gets returned and never used. You want to call the getuserlocation() function with that returned value.

So instead of just calling

.then(getAllUserConnection)

I would do

.then(function(context){getAllUserConnection(context);})

That should make use of the returned value and make sure you are using it.