Problem indexing parent+child in 6.0.1

[scherbone (Sean Cherbone)]

In Elasticsearch (ES) 5.6.4, I was able to index a new parent document along with multiple child documents within a bulk (new children related to new parent). I can no longer do this in 6.0.1. I get the error:

can't specify parent if no parent field has been configured

Here is the relevant part of the (now single) type mapping:

"join_role": {
	"type":"join",
	"relations":{
		"user":["msg","pending_contact","member_contact","non_member_contact"]
	}
}

I then index the parent with:

"join_role": "user"

and the children with:

"join_role": {
      "name": "msg",
      "parent": "<parent_id>"
}

I want to make sure other people are experiencing this before I file a bug report. Thanks in advance.