Lists of RecordSets cannot be typed as Reference List

[vviers]

Problem

Say in table Carts there's a column $Cart of type Reference List, pointing to a Foods table

In a third table (say, Customers), we want to list all Foods that are in a customer's cart.

Carts.lookupRecords(customer=$id).Cart returns a "list of lists" ([Foods[[1, 2, 3]], Foods[[1]]]) which is not handled properly by Grist when trying to type it as a Reference List

A fix is to return a flattened list using a list comprehension [el for val in Carts.lookupRecords(customer=$id).Cart for el in val]

An example document can be found here : https://docs.getgrist.com/bAjQhzJRgBJB/Issue-1130-Example/p/1

Proposed solution

Option 1 : Implement a FLATTEN_RECORDS_LIST function that transforms a list of lists of records into a list of records. We should offer a deduplicate option to this function.

Option 2 : Make Grist handle those list of lists of records as a valid input for the Reference List type

WDYT ?