openUri doesn't flush buffer commands on open (and why createConnection returns a Promise?)

[ValYouW]

mongoose 4.11.0

When using openUri the connection onOpen is not getting called and as a result all collections are in "buffer" mode, maybe the following line should be replaced to _this.onOpen()
https://github.com/Automattic/mongoose/blob/master/lib/connection.js#L739

[Edit] I'll elaborate as I forgot to mention I did a little hack :)
What I was trying to achieve is make createConnection to always return the Connection object, whether it was called with useMongoClient or not. The fact that it returns a Promise is breaking my code as now I need to wait for the Promise to resolve before I can define models on this connection.
So maybe the correct question is "Why createConnection doesn't return the connection but a Promise?", since connection has a buffer mechanism, and it is an emitter that emits an "open" event it make sense to get it immediately (like it was before 4.11.0).

Thoughts?

[agoldis]

I have faced the issue when a query on a collection is executed before mongoose connects.

The code below never executes the find query:

const mongoose = require('mongoose')
mongoose.Promise = Promise
const schema = mongoose.Schema({}, {strict: false});
const Model = mongoose.model('mymodel', schema);

Model.find().exec().then(console.log)

mongoose.connect('mongodb://localhost:27017/testingBug', { useMongoClient: true })
.then(() => {
  console.log('connected')
})
.catch(console.error)

Here's what happens:

1. connection is created using openUri (which is preferred method due to DeprecationWarning: open() is deprecated in mongoose >= 4.11.0, use openUri() instead #5399)
2. connection gets ready state
3. it emits open event
4. nothing really happens.

As @ValYouW suggests, calling _this.onOpen() solves the problem, because it triggers _this.collections[i].onOpen(); for every known collection, and sets collection's buffer to false

@ValYouW I suggest to change the title to something like openUri doesn't change collections buffer mode

UPDATE:
Here's a workaround

mongoose.connect('mongodb://localhost:27017/testingBug', { useMongoClient: true }, function (ignore, connection) {
  connection.onOpen()
}).then(() => {
  console.log('connected')
})
.catch(console.error)

[sobafuchs]

this seems like a legit enhancement to me, i'll add it to the docket. Thoughts @vkarpov15 ?

[vkarpov15]

You're right about the onOpen() part, fixed in 6c025c8. Thanks for reporting this issue and for trying out useMongoClient.

One of the changes for useMongoClient is that createConnection() and connect() return promises that resolve to the connection object if the initial connection succeeds. This is to make mongoose more consistent with the underlying driver and be more graceful with separating out initial connection errors (which are not recoverable) and connection errors that happen after initial collection (which may be recoverable). Right now we have a bunch of dirty hacks to support things like await mongoose.createConnection() and I think the promise-based direction is the right one going forward. If you have any concerns, please voice them here.

[ValYouW]

@vkarpov15 Thank you. Returning Promise from createConnection/connect doesn't give users the option to use the "command buffering" feature of mongoose (which is cool) on app startup (unless it is a single-connection and all operations are done on the default connection).
I will try to describe my scenario briefly: I'm developing an api server that works against 2 different databases (app/backoffice), I store both connections in some "connection manager" and on app startup all the models ask for their connection (either app/backoffice) and define the models on that connection, also on app startup, I have services that runs some queries against the different connections - this all happens on app startup without any need to wait for "ready" events etc, all thanks to the "command buffering" feature. Not being able to use this on startup will make my code a bit more complicated having to wait on connection open in all Models and services...
BTW the "connection manager" also takes core of any initial connection errors and it's not a problem at all...
Here is gist:
https://gist.github.com/ValYouW/b93798390551fa4b8bffaaed427b60cb

[vkarpov15]

Yeah I can see that being frustrating. I suppose we can decorate connections with .then() if they're created with useMongoClient, so you can get the best of both worlds.