QueryMapper does not support $function while converting _id

[gbaso]

Suppose you have a document where _id is defined with a $function, like the following:

{
  "_id": {
    "$function": {
      "body": "some js code that takes no arguments"
      "args": []
      "lang": "js"
    }
  }
}

When invoking QueryMapper#getMappedObject (e.g. when building an aggregation pipeline), given that documentField.isIdField() is true, QueryMapper tries to convert every field of the subdocument to an ObjectId. Strings (that are not valid ObjectIds) are left untouched, but an empty array is converted to null, resulting in:

{
  "_id": {
    "$function": {
      "body": "some js code that takes no arguments"
      "args": null
      "lang": "js"
    }
  }
}

Sending this to mongodb will result in an exception since args is required and must be an array.

[christophstrobl]

Thank you for getting in touch.
If you'd like us to spend some time investigating, please take the time to provide a complete minimal sample (something that we can unzip or git clone, build, and deploy) that reproduces the problem.

Since MongoDB already deprecated Server-side JavaScript it might also make sense to think about an alternative approach.

[gbaso]

I have created spring-data-mongodb-4812 with a reproducible sample.