$accumulator support (MongoDB 4.4)

[dimaip]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?

When using the example from the docs on $accumulator I get a weird error $accumulator missing required argument 'init' even though I clearly define init.

If the current behavior is a bug, please provide the steps to reproduce.

https://gist.github.com/dimaip/17365161ad6e6615cd50bda4b75ad93c

What is the expected behavior?

It should work as the docs describe

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Mongo 4.4, Mongoose 5.10.0, Node 10.18.1

[vkarpov15]

Unfortunately, the MongoDB driver doesn't serialize functions by default, which is a problem for $accumulator. You have two options currently:

1. Use template strings to make your functions into strings:

$accumulator: {
  init: `function() {
    return { count: 0, sum: 0 }
  }`
  // ...

2. Set the serializeFunctions option:

Model.aggregate([/*...*/]).option({ serializeFunctions: true });

We'll add a fix for this in Mongoose 5.10.2 so Mongoose handles this case automatically 👍

[dimaip]

Thanks so much for the response and the fix!