Fix Broken Multilist With Search Field Sorting in SitecoreAI

using Sitecore.ContentSearch;
using Sitecore.ContentSearch.ComputedFields;

namespace XmCloudNextJsStarter.ContentSearch.ComputedFields
{
    /// <summary>
    /// Computed index field that produces a stable, case-insensitive alpha
    /// sort key for every item.
    ///
    /// Value precedence:
    ///   1. The item's Display Name (the "__Display name" standard field) when set.
    ///   2. Falls back to the raw Item Name when Display Name is empty.
    ///
    /// Sitecore's <see cref="Sitecore.Data.Items.Item.DisplayName"/> already applies
    /// that fallback, so this class only lower-cases the result to make Solr sorting
    /// case-insensitive.
    ///
    /// Registered with returnType="string" so Solr stores it untokenized and sortable,
    /// which is what "SortField=sortname[asc]" in Multilist with Search sources relies on.
    /// </summary>
    public class SortNameField : IComputedIndexField
    {
        public string FieldName { get; set; }

        public string ReturnType { get; set; }

        public object ComputeFieldValue(IIndexable indexable)
        {
            var scIndexable = indexable as SitecoreIndexableItem;
            var item = scIndexable?.Item;
            if (item == null)
            {
                return null;
            }

            var displayName = item.DisplayName;
            // ToLowerInvariant gives case-insensitive ordering only. It is NOT
            // locale-aware collation, so diacritics are not folded (e.g. "e" and
            // "é" do not sort together). Adequate for the alpha requirement.
            return string.IsNullOrWhiteSpace(displayName)
                ? null
                : displayName.ToLowerInvariant();
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <indexConfigurations>
        <defaultSolrIndexConfiguration>
          <documentOptions>
            <!--
              Universal alpha sort key for Multilist with Search fields.

              SortNameField returns the item's Display Name (falling back to the
              raw Item Name when Display Name is empty), lower-cased for
              case-insensitive ordering.

              returnType="string" maps it to an untokenized Solr string field so it
              is sortable via "SortField=sortname[asc]" in field source queries.
            -->
            <fields hint="raw:AddComputedIndexField">
              <field fieldName="sortname" returnType="string">XmCloudNextJsStarter.ContentSearch.ComputedFields.SortNameField, XmCloudNextJsStarter</field>
            </fields>
          </documentOptions>
        </defaultSolrIndexConfiguration>
      </indexConfigurations>
    </contentSearch>
  </sitecore>
</configuration>

{
  "postActions": {
    "actions": {
      "populateSchema": {
        "indexNames": [
          "sitecore_master_index"
        ]
      },
      "reindex": {
        "indexNames": [
          "sitecore_master_index"
        ]
      }
    },
    // If you have a lot of items, you may need to add/adjust the taskTimeOut
    // See https://doc.sitecore.com/sai/en/developers/sitecoreai/deploying-sitecoreai/the-sitecoreai-build-configuration.html#change-the-post-action-timeout
    "taskTimeOut": "01:00:00"
  }
}