mongodb update not working to replace a document (but $set does) - mongodb

Question

From homework 2_2 of the M101JS course at university.mongodb.com.
Write a program that finds the document with the highest recorded temperature for each state, and adds a "month_high" field for that document, setting its value to true.
The following code should work to update the correct doc in the collection adding "month_high". And the problem is here:
//db.collection('data').update(query, setMonthHigh, function(err, updated) {//OK
db.collection('data').update(query, doc, function(err, updated) {// NOT OK, dunno why
The 2nd update line does not save anything. But if I use the commented update above, it works.
Does anyone have an idea why the 2nd update line isn't working (the one that replaces the whole doc)?
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
if(err) throw err;
var options = { 'sort' : [ ['State', 1], ['Temperature', -1] ]};
var lastDoc = null;
var setMonthHigh = {'$set':{'month_high':true}};
var cur = db.collection('data').find({}, {},options);
//var cur = db.collection('data').find({'month_high':true}, {},options);
cur.each(function(err, doc) {
if(err) throw err;
if(doc == null) {
console.log("Document done");
//return db.close();
db.close();
process.exit(0);
}
var query = {'_id' : doc._id};
//db.collection('data').update({'month_high':true}, {$unset:{'month_high':1}}, function(err, updated) {});
//console.dir(doc);
if ( ( lastDoc != null && lastDoc.State != doc.State ) ||
lastDoc == null )
{
doc['month_high'] = 'true';
//db.collection('data').update(query, setMonthHigh, function(err, updated) {//OK
db.collection('data').update(query, doc, function(err, updated) {
// NOT OK, dunno why
if(err) throw err;
console.dir("Update "+updated);
});
console.dir(doc);
}
lastDoc = doc;
});
});
Also, I'm told that using process.exit(0) is a bad hack, but it was the only way I could exit the code without throwing exceptions on return db.close();. If anyone knows a way to exit the code gracefully, that would be great too.

Answer 1

First of all, you have made a mistake in part of your code responsible for updating a database.
db.collection('data').update is an asynchronous function. It means that you cannot execute db.close() without knowing if all updates have been completed. For more details please read: How can I use a cursor.forEach() in MongoDB using Node.js?
Both updates works. However there is one difference between two of them:
db.collection('data').update(query, setMonthHigh, function(err, updated)
updates records with a new field: 'month_high': true <- boolean value
Query to find these records is: db.data.find({'month_high': true})
db.collection('data').update(query, doc, function(err, updated)
updates records with a new field: 'month_high': 'true' <- string value
Query to find these records is: db.data.find({'month_high': 'true'})
Your updated code:
Before run it, first install async library (npm install async)
var async = require('async');
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
if(err) throw err;
var q = async.queue(function (doc, callback) {
db.collection('data').update({_id: doc._id}, doc, callback);
}, Infinity);
var options = { 'sort' : [ ['State', 1], ['Temperature', -1] ]};
var lastDoc = null;
var cur = db.collection('data').find({}, {},options);
cur.each(function(err, doc) {
if(err) throw err;
if(doc === null) return;
if ( ( lastDoc !== null && lastDoc.State !== doc.State ) ||
lastDoc === null )
{
doc['month_high'] = true;
q.push(doc, function(err, updated) {
if(err) throw err;
console.log("Update "+updated);
});
}
lastDoc = doc;
});
q.drain = function() {
if (cur.isClosed()) {
console.log('Document done');
db.close();
}
}
});