When do you call db.close() when using cursor.forEach()?

Question

If using .toArray(), I know you could use db.close() inside the callback like so:

db.collection('grades').find(query).toArray(function(err, docs) {
  if (err) throw err;
  console.dir(docs);
  db.close();
});

But what if you need to iterate over the array? It seems excessive to 1) iterate through the cursor to construct the array and then 2) iterate over the array to do your work. So I was thinking that it'd be good to use cursor.forEach().

var MongoClient = require('mongodb').MongoClient;

MongoClient.connect('mongodb://localhost:27017/weather', function(err, db) {
  if (err) throw err;

  var cursor = db
                .collection('data')
                .find({})
                .sort({State: 1, Temperature: -1});

  var prevState = '';
  var curr;

  cursor.forEach(function(doc) {
    if (doc.State === prevState) return;

    db.collection('data').update(doc, { $set: { month_high: true}});
    prevState = doc.State;
  });

  db.close();
});

However, this gives me an error:

~/code/m101js $ node test.js
/Users/azerner/code/node_modules/mongodb/lib/utils.js:97
    process.nextTick(function() { throw err; });
                                        ^
TypeError: undefined is not a function
    at /Users/azerner/code/node_modules/mongodb/lib/cursor.js:527:15
    at handleCallback (/Users/azerner/code/node_modules/mongodb/lib/utils.js:95:12)
    at /Users/azerner/code/node_modules/mongodb/lib/cursor.js:493:22
    at handleCallback (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:234:5)
    at /Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:533:22
    at queryCallback (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:167:20)
    at Callbacks.flush (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:86:7)
    at Server.destroy (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:694:41)
    at Server.close (/Users/azerner/code/node_modules/mongodb/lib/server.js:344:17)
    at Db.close (/Users/azerner/code/node_modules/mongodb/lib/db.js:273:19)
~/code/m101js $ node test.js
/Users/azerner/code/node_modules/mongodb/lib/utils.js:97
    process.nextTick(function() { throw err; });
                                        ^
TypeError: undefined is not a function
    at /Users/azerner/code/node_modules/mongodb/lib/cursor.js:527:15
    at handleCallback (/Users/azerner/code/node_modules/mongodb/lib/utils.js:95:12)
    at /Users/azerner/code/node_modules/mongodb/lib/cursor.js:493:22
    at handleCallback (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:234:5)
    at /Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:533:22
    at queryCallback (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/cursor.js:167:20)
    at Callbacks.flush (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:86:7)
    at Server.destroy (/Users/azerner/code/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:694:41)
    at Server.close (/Users/azerner/code/node_modules/mongodb/lib/server.js:344:17)
    at Db.close (/Users/azerner/code/node_modules/mongodb/lib/db.js:273:19)
~/code/m101js $

It seems to be all library code and I'm not really sure how to debug it. But it looks like the problem is in db.close().

Is this so?
If so, then where else would I put db.close()?

@adeneo I'm not sure. I'm actually taking a [class](https://university.mongodb.com/courses/10gen/M101JS/2015_May/syllabus) and am not quite sure what I'm doing. But unless closing it is something you'd never want to do in this scenario, my question remains. — Adam Zerner, Jun 05 '15 at 21:03
Generally speaking, I think that most applications would just connect once, and keep the connection open, there's really no need to close the connection for each call to the DB — adeneo, Jun 05 '15 at 21:08
This isn't really an application. It's a homework assignment that is just making this one call. — Adam Zerner, Jun 05 '15 at 21:10
I'm guessing the `forEach` call is async, and `update` surely is, so you're probably closing the connection while you're still using it. — adeneo, Jun 05 '15 at 21:29
Please look at this SO answer: http://stackoverflow.com/questions/25507866/how-can-i-use-a-cursor-foreach-in-mongodb-using-node-js . I think it should help. — , Jun 05 '15 at 21:38

score 2 · Accepted Answer · answered Jun 06 '15 at 03:05

The issue is that db.close() was being called, and then I was trying to interact with the database. You can't interact with the database when the connection has already been closed.

The reason why my code was trying to interact with the database after the connection was closed is because the .update() is asynchronous.

Using async.each worked for me. async.each gives you a callback to run after everything in the collection has been iterated through. I called db.close() in that callback, which is the time when I wanted to close the connection to the database.

When do you call db.close() when using cursor.forEach()?

1 Answers1