We had a bug when selecting fields with names containing dollar signs or dots, or when selecting fields using aliases containing those characters. What happened is the mongodb query plan would produce a stage like this:

{
  "$replaceWith": {
    "foo.bar": "$title"
  }
}

The use of "foo.bar" leads to this error:

MongoServerError: FieldPath field names may not contain '.'. Consider using $getField or $setField.

The fix is, as the error suggests, to use the $setField operator which takes a field argument that is not evaluated as an expression, and is therefore allowed to contain arbitrary characters.

{
  "$replaceWith": {
    "$setField": {
      "field": "foo.bar",
      "value": "$title",
      "input": "$$CURRENT"
    }
  }
}

But that leads to an issue when we want to select multiple fields: in the first snippet the argument to $replaceWith was an object literal where we could set multiple fields. OTOH { "$setField": { ... } } is an expression that produces an object with a single field, and does not accept more keys. To select multiple fields with escaping we have to nest calls to $setField for every selected field. For example,

{
  "$replaceWith": {
    "$setField": {
      "field": "foo.bar",
      "value": "$title",
      "input": {
        "$setField": {
          "field": "year",
          "value": "$year",
          "input": {
            "$setField": {
              "field": "rated",
              "value": "$rated",
              "input": "$$CURRENT"
            }
          }
        }
      }
    }
  }
}

So that is what this PR does.