Sure, here is a simplified version of my actual collections:

{
  "name": "parent",
  "fields": [
    { "name": "parent_id", "type": "string" },
    { "name": "name", "type": "string" },
  ]  
}

{
  "name": "children",
  "fields": [
    { "name": "child_id", "type": "string" },
	{ "name": "name", "type": "string" },
    { "name": "parent_id", "type": "string", "reference": "parent.parent_id" }   
  ]
}

Based on the current join documentation, it doesn't seem like you can use query_by with a reference collection at all (if I try it hangs), but I'd like to do something like:

{
  "q": "abc",
  "query_by": "name,$parent(name)",
  "collection": "children"
}

A simple solution in my case seems to be just to add all parent fields directly to the children, and have a single collection with the children alone. I can then query across all the fields I want and use group by to aggregate and limit on the parent ID

// collection
{
  "name": "children",
  "fields": [
    { "name": "child_id", "type": "string" },
	{ "name": "name", "type": "string" },
    { "name": "parent_id", "type": "string" },
    { "name": "parent_name", "type": "string" }    
  ]
}

// query
{
   "q": "abc",
   "query_by": "name,parent_name",
   "collection": "children",
   "group_by": "parent_id",
   "group_limit": 5
}

The only downside I think is that I have to duplicate the parent information across many children, but the collection isn't that large so I don't think this is an issue.