Updated related objects in Loopback behaving inconsisently

Question

When updating a related object, I get two completely different behaviors from the same code. For example, a question belongs to a student. I run the code below on two separate machines.

Question.observe('after save', function (ctx, next) {

   var question = ctx.instance

   question.student(function (err, student) {
      student.updateAttributes({points: student.points - question.points})
   })

next();

})

On the first machine it works perfectly. So I thought I was done. But then when I run the same exact code on a different machine I get the following error:

  student.updateAttributes({points: student.points - question.points})
                                           ^
  TypeError: Cannot read property 'points' of null

I am using the memory connector

  "db": {
     "name": "db",
     "connector": "memory",
   }

The only thing I an think of is that I have two different versions of loopback (one with a bug and the other one without) ... but then package.json is exactly the same for both also!?

{
  "name": "foo",
  "version": "1.0.0",
  "main": "server/server.js",
  "scripts": {
    "start": "node .",
    "pretest": "jshint .",
    "test": "mocha"

  },
  "dependencies": {
    "compression": "^1.0.3",
    "cors": "^2.5.2",
    "loopback": "^2.22.0",
    "loopback-boot": "^2.6.5",
    "loopback-component-explorer": "^2.1.0",
    "loopback-datasource-juggler": "^2.39.0",
    "serve-favicon": "^2.0.1"
  },
  "devDependencies": {
    "chakram": "^1.2.1",
    "jshint": "^2.8.0",
    "loopback-testing": "^1.2.0",
    "mocha": "^2.3.4",
    "mocha-bamboo-reporter": "^1.1.0"
  },
  "repository": {
    "type": "",
    "url": ""
  },
  "description": "foo"
}

score 1 · Answer 1 · edited May 23 '17 at 12:07

1

Your packages.json specifies that a package should be higher than a set version, so you could have one machine where the version for loopback is v2.23.0 and one at v2.26.2 (the current latest), if you ran npm install at different times.

You can run npm list on both machines to compare the versions that are installed.

Run npm update to update packages.

To update your package.json to use the most up to date dependencies, check out this answer.

edited May 23 '17 at 12:07

Community

1
1

answered Jan 06 '16 at 10:28

conradj

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Maybe he could run `npm loopback -v` to check versions of loopback – Overdrivr Jan 06 '16 at 10:33
@Overdrivr `npm list` for all dependencies (I just chose loopback as an example as they are all specified in the same way. Thanks for the comment, have added to answer. – conradj Jan 06 '16 at 12:24

score 1 · Accepted Answer · answered Jan 06 '16 at 10:30

You are not checking for errors in question.student. So first you need to fix that.

Then, probably not completely related, but question.student is most likely asyncronous, so you are calling next before question.student is finished.

A better code would look like this

Question.observe('after save', function (ctx, next) {

   var question = ctx.instance

   question.student(function (err, student) {
     if (err) return next(err);
       
     student.updateAttributes({points: student.points - question.points})
         
     next();
   });
});

Also, I don't know anything about updateAttributes but if it is async you also need to call next() only once it has finished (using a callback function like you did for question.student for example), and check for errors as well.

ALWAYS check errors. It's more than just a good practice.

Updated related objects in Loopback behaving inconsisently

2 Answers2