@Florian R. In the latest RC build you will not get this error > Invalid Format: Request failed with HTTP code 400 | Server said: Could not find a reference for doc 0 You can sort by like:

{
  "collection": "candidatures",
  "q": "*",
  "filter_by": "$annonces(id:*) || $structures(id:*)",
  "sort_by": "$structures(nom:asc)"
}