How to implement $$ROOT

How one could implement $$ROOT variable as in here https://docs.mongodb.com/manual/reference/operator/aggregation/group/#group-documents-by-author?

It’s in mongo since 2.6.

Do ‘custom operators’ fit for this?

I tried to implement it this way, but it doesn’t work.

Mingo.addOperators(Mingo.OP_PIPELINE, function (m) {
  return {
    $$ROOT: function (obj) {
      return obj;
    }
  }
})

It throws the error: “Invalid operator name ‘$$ROOT’”

[kofrasa]

I have implemented $redact on the development branch. Looking good so far and should be out in the next version. With regards to the system variables they can be handled directly within the value resolution functions. I don’t think it is a good idea to allow defining new ones at present.

After careful consideration, I still found the ones available to $redact more useful though $$ROOT can be implemented quite easily. $$CURRENT can also be handled but will require a way to rebind the variable as per the documentation.

@zag2art Out of curiousity, what is your use case for requiring $$ROOT?

[kofrasa]

Still some more work to do then. My implementation did not handle subpaths for $$ROOT.

I will make the update in the next minor release. Thanks for the tests.