$out in aggregation stagereturns unknown error

[enjoywithouthey (Justin Enjo)]

Using Atlas aggregation wizard on a sample collection, the $out stage returns

An unknown error occurred

When running the aggregation inside a function, the result is:

(AtlasError) $out is not allowed or the syntax is incorrect

Code here:

exports = function(payload, response) {
  const mongodb = context.services.get("mongodb-atlas");
  
  const readColl = mongodb.db("temp_db").collection("temp_read")
  const writeColl = mongodb.db("temp_db").collection("temp_write")
  
  const myAgg = readColl.aggregate([ { $sample: { size: 2 } }, { $out: "temp_write" } ])
  return myAgg
}

Any advice from the community?

[MaBeuLux88 (Maxime Beugnet)]

Hi @enjoywithouthey,

Can you share the pipeline and which sample collection you used?

Also which cluster Tier are you using in Atlas and which version ? Anything out of the ordinary with your cluster?

[enjoywithouthey (Justin Enjo)]

My Tier is $0.1/1M Serverless

The sample data I made up. It’s in a collection I’m calling temp_read

I’m selecting some random documents and pushing them to an empty collection called temp_write

$merge in a previous pipeline worked fine. $out is giving errors.

Here’s the sample data:

[
    {
      "1": "A"
    },
    {
      "2": "B"
    },
    {
      "3": "C"
    },
    {
      "4": "D"
    },
    {
      "5": "E"
    },
    {
      "6": "F"
    },
    {
      "7": "G"
    }
]

Pipeline:

[
  {
    $sample:
      {
        size: 2,
      },
  },
  {
    $out:
      "temp_write",
  },
]

Thanks for your help!

[enjoywithouthey (Justin Enjo)]

[enjoywithouthey (Justin Enjo)]

@MaBeuLux88 I did a little digging and found something interesting…

On some previous Clusters I made for tutorials, version 5.0.15 installs automatically under the M0 free tier. No problems with $out.

I made a recent paid production M1 Cluster (the cluster causing the problem with $out) and that cluster is version 6.2.1. The problem with $out persists.

Hope this helps.

[enjoywithouthey (Justin Enjo)]

@MaBeuLux88 to verify, I just spun-up a M10 Dedicated Cluster, which allowed me to choose v5 in the advanced configuration (instead of v6).

$out works no problem.

[MaBeuLux88 (Maxime Beugnet)]

I was able to reproduce on a serverless instance as well !

[MaBeuLux88 (Maxime Beugnet)]

Serverless instances don’t support the $out stage. Use $merge instead.

Serverless Instance Limitations — MongoDB Atlas

[enjoywithouthey (Justin Enjo)]

Ok, thanks.

Maybe that we’ve created a thread on this subject, the results will be scrapped and show up in search engines. Appreciate the help.

[MaBeuLux88 (Maxime Beugnet)]

Definitely!

I wasn’t aware of this limitation of serverless. Hopefully it goes away soon.
But the error message could be better. I’ll create a ticket to improve it. It would have been easier for you to understand the source of the problem.

Cheers,
Maxime.