Javascript function does not produce any output

Working with Data
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1

This topic comes from Using $function to run a javascript function is not working in mongosh - #2 by Jason_Tran

I need to use a javascript function inside an aggregation, so I started from basic aggregation, like “Hello” functions (reference: How to Use Custom Aggregation Expressions in MongoDB 4.4 | MongoDB)
In previous topic I learnt that I need to use standalone shell, not compass shell, to run these kind of aggregations (If I use Compass shell, the code hangs and the prompt is not returned). I have 2 sintaxes for this simple code, but they don’t produce any output. Do I need to install anything else so these codes work?

db.movies.aggregate([
    { 
      $addFields: {
        fromFunction: {
          $function: {
            body: "function(){return 'hello'}",
            args: [], 
            lang: 'js'
          }
        }
       }
     }
  ])
  1. use test - Switch to "test" database
  2. db.movies.insertOne({a:1}) - Insert a sample / test document into the "movies" collection
db.movies.aggregate([
  {
    '$addFields': {
      fromFunction: { '$function': { body: function(){return 'hello'}, args: [], lang: 'js' } }
    }
  }
])
2

Hi @Luis_Leon

I tried the exact code you posted and got the expected output:

> db.version()
6.0.6

> db.test.find()
[ { _id: 0 } ]

> db.test.aggregate([
...     {
...       $addFields: {
...         fromFunction: {
...           $function: {
...             body: "function(){return 'hello'}",
...             args: [],
...             lang: 'js'
...           }
...         }
...        }
...      }
...   ])
[ { _id: 0, fromFunction: 'hello' } ]

I literally just copy pasted the first example you have. With regards to your second example, the function(){...} is unquoted. Perhaps this is the issue?

Having said that, note that using $function will be a lot less performant compared to using built-in aggregation pipelines. It’s recommended for you to do this if your needs cannot be satisified otherwise.

Best regards
Kevin

1 Like
3

Hi @kevinadi Thanks for your reply. Did you do anything else in your mongo installation in order to run a function? I tried again, I’m using the exact mongo version but I’m not getting an output like yours. Anyway, you mentioned that a built-in aggregation pipeline has better performance. The reason why I’m trying to use a function is parsing a json string, I haven’t find any other way to parse that kind of data field without using a function. Can you think an alternative way to do it?

4

Hi @Luis_Leon

No I didn’t do anything special. It’s just a plain vanilla MongoDB 6.0.6 single-node replica set deployment that I use locally for testing purposes. However, it shouldn’t matter if it’s a standalone node, a replica set, or even a sharded cluster.

However if you’re using Atlas shared tier (M0/M2/M5) then server-side Javascript is not supported (see Atlas M0 (Free Cluster), M2, and M5 Limitations). Otherwise you’ll see an error like MongoServerError: $function not allowed in this atlas tier

Can you show us an example document, and how the output should look like?

Best regards
Kevin

5

Sure. Look at this link, it uses real data I’m working with. I don’t know why it works in this mongo playground but is not working in my environment (I didn’t create this link but some guy in stackoverflow):

6

Hi @Luis_Leon

Thanks for providing the example.

I see that the javascript function executed is:

function(jsonString){return JSON.parse(jsonString)}

basically it just parses a JSON document that was stored as a string, and dump the parsed document in the output, creating a sub-document.

Wouldn’t this be better if the JSON string is stored as an actual sub-document in MongoDB? After all, this is what MongoDB is good at. Depending on your case, maybe you can pre-process the raw workflowParams string during document insertion, so you don’t need to use $function in the query.

Also, I copy pasted the example document and the query, and the output I get is identical to the MongoPlayground output.

Best regards
Kevin

3 Likes
7

Thanks Kevin, that’s a good observation, I don’t know if the customer is willing to pre-process the raw string, but maybe is the only solution to this problem for now, as I checked and noticed that they have mongo 4.0.0, and apparently $function is not supported in that mongodb version (my local installation is Mongo 6.0.6).

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