ERROR IN AGGREGATE EXERCISE

[mojoboss]

This is my solution for the aggregate exercise, the problem is its giving this error-

var size = process.argv[2];
var url = "mongodb://localhost:27017/learnyoumongo";
var mongo = require("mongodb").MongoClient;
mongo.connect(url, function(err, db){
if(err) throw err;
var collection = db.collection("prices");
collection.aggregate({$match: {"size": size}}, {$group: {_id: "average"}, average:{$avg: "$price"}}).toArray(function(err, results){
if(err) throw err;
console.log(Number(results[0].average).toFixed(2));
db.close();
});
});

error-

/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/exercises/aggregate/exercise.js:34
db.collection('prices').remove({}, function(err) {
^
TypeError: Cannot read property 'collection' of undefined
at Exercise. (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/exercises/aggregate/exercise.js:34:5)
at next (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:260:17)
at Exercise.end (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:266:5)
at Workshopper.end (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper/workshopper.js:191:12)
at Workshopper.done (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper/workshopper.js:323:19)
at Exercise. (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:149:14)
at /home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:136:16
at Exercise. (/home/ubuntu/.nvm/versions/node/v4.1.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/filecheck.js:10:14)
at FSReqWrap.oncomplete (fs.js:82:15)

I'm not even using .remove(), but it still gives the same error. I used the official solution but still getting the same error.

[eliotn]

I would like to say that I got the exact same error just now. Is the database being initialized properly?

[Atomk]

Note: I understand this is an old issue, people still use this workshopper though, so...

@mojoboss This is not a bug, there is a lot going on in this lesson and it's easy to miss something, your solution has a couple of issues.

First, your $group object has a weird structure, you wrote (I formatted it to make it easier to see the difference):

{
  $group: {_id: "average"},
  average: {$avg: "$price"}
}

but it should be:

{
  $group: {
    _id: "average",
    average: { $avg: "$price" }
  }
}

Second, the parameter of the aggregate function should be an array of objects, while you passed two objects. What you did:

collection.aggregate(
  { $match: { ... }},
  { $group: { ... }}
)

The correct way (the objects are now elements of an array, notice the square brackets):

collection.aggregate([
  { $match: { ... }},
  { $group: { ... }}
])

There you have it! After these fixes everything works correctly on my machine.
@evanlucas I think you can close this issue.