[Subquery] Transform Documents across Collections

[Norbert Kutasi]

Hello,

We have been using [subquery] to come up with arbitrary complex hierarchies
in our document responses.

It works well as long as the documents are in the same collection however
based on the reference guide I infer it can bring in documents from
different collections except it throws an error.
https://lucene.apache.org/solr/guide/8_2/transforming-result-documents.html#subquery


We are on SOLR 8.2 and in this sandbox we have a 2 node SOLRCloud cluster,
where both collections have 1 shard and 2 NRT replicas to ensure nodes have
a core from each collection.
Basic Authorization enabled.

Simple steps to reproduce this issue in this 2 node environment:
./solr create -c Collection1 -s 1 -rf 2
./solr create -c Collection2 -s 1 -rf 2

Note: these collections are schemaless, however we observed the ones with
schemas.

Collection 1:
<add>
   <doc>
      <field name = "id">1</field>
      <field name = "first name">John</field>
   </doc>
   <doc>
      <field name = "id">2</field>
      <field name = "first name">Peter</field>
   </doc>
</add>

Collection 2:
<add>
   <doc>
      <field name = "id">3</field>
      <field name = "first name">Thomas</field>
 <field name = "reporting_to">2</field>
   </doc>
   <doc>
      <field name = "id">4</field>
      <field name = "first name">Charles</field>
      <field name = "reporting_to">1</field>
   </doc>
   <doc>
      <field name = "id">5</field>
      <field name = "first name">Susan</field>
 <field name = "reporting_to">3</field>
   </doc>
</add>


http://localhost:8983/solr/Collection1/query
{
  params: {
    q: "*",
    fq: "*",
    rows: 5,
fl:"*,subordinate:[subquery fromIndex=Collection2]",
    subordinate.fl:"*",
    subordinate.q:"{!field f=reporting v=$row.id}",
    subordinate.fq:"*",
    subordinate.rows:"5"
  }
}

{
  "error":{
    "metadata":[
      "error-class","org.apache.solr.common.SolrException",
      "root-error-class","org.apache.solr.common.SolrException"],
    "msg":"while invoking subordinate:[subqueryfromIndex=Collection2] on
doc=SolrDocument{id=stored,indexed,tokenized,omitNorms,indexOptions=DOCS<id:1>,
first_name=[stored,index",
    "code":400}}


Where do we make a mistake?

Thank you in advance,
Norbert