No event emit while createConnection

[amustaque97]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

In current version of package, connection is not able to emit event while calling createConnection mongoose function. This is happening after merging PR #1083 and similar changes in other area of code.

In order to test this bug, I reverted the changes locally and then it started working as expected. All connectionStates were working fine but then I started facing issue regarding InjectConnection which you already solved with the mentioned PR #1083. I feel this is a bit tricky to fix.

Minimum reproduction code

https://github.com/amustaque97/nestjs-mongoose-no-event

Steps to reproduce

1. npm i
2. npm start:dev
3. You will see line number 11 printed https://github.com/amustaque97/nestjs-mongoose-no-event/blob/main/src/app.module.ts#L11
4. connected will not be printed on console https://github.com/amustaque97/nestjs-mongoose-no-event/blob/main/src/app.module.ts#L12

Expected behavior

After running server, it should print both the console statements.

Package version

9.0.3

mongoose version

6.2.7

NestJS version

8.0.0

Node.js version

16.13.2

In which operating systems have you tested?

• macOS
• Windows
• Linux

Other

No response

[jmcdo29]

What's the reason for wanting to listen for the connection event? With asPromise() mongoose doesn't resolve the promise until the connection succeeds (connection state 1) or rejects when it fails (connection state 0)

[amustaque97]

okay! There could be some cases where in based on the events we need to perform some operation for example: once mongoose connection is successful we are opening some stream and when mongoose connection is disconnected. We want to close the opened stream. We want to manipulate stream based on the connection event...

I actually tried with readyState variable but it is not realtime.

[jmcdo29]

Do you have access to these variables with just mongoose? Or in otherwords, is it our wrapper that is causing an issue, or does mongoose 6 (when the asPromise() was added) work slightly different than previous versions?

[amustaque97]

Yeah, till version 5 it was straightforward to implement. For version 6 I tested with the below code and it seems to be working.. I have attached screenshot for reference. In the screenshot, I'm using mongodb docker instance.

const mongoose = require('mongoose');

(async() => {
    const conn = mongoose.createConnection('mongodb://localhost:9972');
    conn.on('connected', function(){ console.log('Hi')});
    await conn.asPromise();
    conn.on('disconnected', function(){ console.log('Bye')});
})();

PS: If I write like this await mongoose.createConnection(uri, mongooseOptions).asPromise() event emitter doesn't work. So even I'm not 100% sure whether it's an intended feature or @nestjs/mongoose wrapper is causing an issue.

[jmcdo29]

I think this is a side effect of how mongoose v6 is supposed to be used. Might want to try bringing it up with them and seeing what they say about it.

[amustaque97]

I think this is a side effect of how mongoose v6 is supposed to be used. Might want to try bringing it up with them and seeing what they say about it.

I have created an issue there. Automattic/mongoose#11545

[kamilmysliwiec]

Let's track this here Automattic/mongoose#11545

[juona]

This should be reopened IMO. There is a problem here:

return await lastValueFrom(
  defer(async () =>
    mongooseConnectionFactory(
      await mongoose
        .createConnection(uri as string, mongooseOptions)
        .asPromise(),
      mongooseConnectionName,
    ),
  ).pipe(handleRetry(retryAttempts, retryDelay)),
);

Quoting the maintainer of mongoose:

The problem is that const conn = await mongoose.createConnection('mongodb://localhost:27017').asPromise(); waits for Mongoose to finish connecting. So Mongoose is already connected when you register an event listener. The event listener works, you're just registering it too late.

So the consumer does not really have any way to listen for connection events.