For labelled single nested child documents update functionality gives error for xml format

[eastern-ravindra]

Solarium version(s) affected: 6.3.1 Solr version: 8.6.3 Solr mode: standalone / cloud

Description

For labelled single nested child documents if passed multidimension array for update functionality gives missing required field issues for xml format

Solr HTTP error: OK (400) { "responseHeader":{ "status":400, "QTime":183}, "error":{ "metadata":[ "error-class","org.apache.solr.common.SolrException", "root-error-class","org.apache.solr.common.SolrException"], "msg":"[doc=null] missing required field: title", "code":400}}

How to reproduce

If we try to update solr index with below data where title is required field and have parent child relationship

$data = [
    "id" => "parent1",
    "title" => "Parent Document",
    "children" => [
        [
            "id" => "child1",
            "title" => "Child Document 1"
        ],
        [
            "id" => "child2",
            "title" => "Child Document 2"
        ]
    ]
];

While executing update query it gives missing required field title.

Possible Solution

} else {
         $xml .= '<doc';
         if ('_childDocuments_' !== $key) {
             // labelled single nested child documents can't be indexed in XML, but
             // we aim for forward compatibility with the proposed syntax in SOLR-16183
             $xml .= ' name="'.$key.'"';
         }
         $xml .= '>';
         foreach ($value as $k => $v) {
             $xml .= $this->buildFieldsXml($k, $boost, $v, null);
         }
         $xml .= '</doc>';
}

Above code in src/QueryType/Update/RequestBuilder/Xml.php file line no. 306 will create xml output as follows

<doc>
	<field name="id">parent1</field>
	<field name="title">Parent Document</field>
       <doc name="children">
         <doc>
            <field name="id">child1</field>
	    <field name="title">Child Document 1</field>
        </doc>
       <doc>
            <field name="id">child2</field>
	    <field name="title">Child Document 2</field>
        </doc>
       <doc>
</doc>

<doc name="children"> is this section really needed? Ideally xml should be like

<doc>
	<field name="id">parent1</field>
	<field name="title">Parent Document</field>
   
         <doc>
            <field name="id">child1</field>
	    <field name="title">Child Document 1</field>
        </doc>
       <doc>
            <field name="id">child2</field>
	    <field name="title">Child Document 2</field>
        </doc>
       
</doc>

I think, we should modify foreach loop in above code as below to fix this issue(just a suggestion)

foreach ($value as $multivalue) {
    foreach ($multivalue as $k => $v) {
        $xml .= $this->buildFieldsXml($k, $boost, $v, null);
    }
}