ETL: create edges with multiple joinFieldName

[JHime]

Hello, I have a class "Invoice" with unique index on fields "month" and "cardNumber".

I have another class "Item", that have many fields, plus "month" and "cardNumber".

When I run the "Item" ETL, I need to create edges from "Invoice" to "Item", but I cannot use the "joinFieldName" property with multiple fields.

I am trying the code below (and many other changes):

    { "edge": { "class": "HasItems",

                "joinFieldName": "month,cardNumber",

                "lookup": "Invoice.month, Invoice.cardNumber",

                "direction": "in"

            }

        }

but it seems the ETL only allows single-field relationships.

Is that true?

Is there a way to create edges on such tables (multiple-fields key) during ETL? If not, how to create the edges after the bulk load (with SQL)?

Thanks in advance,

JH