How do I iterate over all fields of a document (or a sub-document) when applying an aggregation?

Working with Data
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1

Hello, I want to do something that seemed trivial to me, but I can’t find a way to do it in mongodb aggregations. I’m using Compass to make my aggregations.

Let’s say I have this aggregation

$group: {
  {
    _id: '$region',
    "customer_average_income": {
      $avg: '$customer_income'
    },
    "customer_average_expenses": {
      $avg: '$customer_expenses'
    },
    "customer_average_something": {
      $avg: '$customer_something'
    }
  } 
}

It works fine, but I have to hard-code the fields of which I want to calculate the average, manually. Let’s say I have many of those fields of which I want to calculate the average, how do I achieve an equivalent aggregation where I can just pass the field as a variable?

Something like this, but $map cannot be used in $group aggregation:

$group: {
  {
    _id: '$region',
    "$map": {
        input: '$$ROOT',
        in: {$avg: '$$ROOT'}
    }
}

What am I missing? Any help will be very appreciated, thank you.

3

In JS you could do:

field_to_average = "customer_something"

$group : {
  _id : "$region" ,
  [ field_to_average + "_average" ] : {  $avg : "$" + field_to_average } 
}
5

Well, that involves using code to build the aggregation, I already thought about that. But…
Is that a good practice with mongoDB? The view would not update automatically if I added new fields, for example, unless you run the code again, am I right?