I am unable to get the following code to work. I was trying to do exactly the same a Paul has done in his blog, only for V11.

https://codeshare.co.uk/blog/how-to-search-by-picked-multi-node-tree-picker-values-in-umbraco-v8/

I have IConfigureNamedOptions set up and have debugged the code and it is getting hit.

public class ConfigureExternalIndexOptions : IConfigureNamedOptions<LuceneDirectoryIndexOptions>
{
    public void Configure(string? name, LuceneDirectoryIndexOptions options)
    {
        if (name is global::Umbraco.Cms.Core.Constants.UmbracoIndexes.ExternalIndexName)
        {
            options.FieldDefinitions.AddOrUpdate(new FieldDefinition("blogDate", FieldDefinitionTypes.DateTime));
            options.FieldDefinitions.AddOrUpdate(new FieldDefinition(Constants.ExamineFields.SearchableBlogCategories, FieldDefinitionTypes.FullText));
        }
    }

    // Part of the interface, but does not need to be implemented for this.
    public void Configure(LuceneDirectoryIndexOptions options)
    {
        throw new System.NotImplementedException();
    }
}

I then have a IComponent which gets the values and adds them to the index to the new field.

public class IndexerComponent : IComponent
{
    private readonly IExamineManager _examineManager;
    private readonly IUmbracoContextFactory _umbracoContextFactory;

    public IndexerComponent(IExamineManager examineManager,
        IUmbracoContextFactory umbracoContextFactory)
    {
        _examineManager = examineManager ?? throw new ArgumentNullException(nameof(examineManager));
        _umbracoContextFactory =
            umbracoContextFactory ?? throw new ArgumentNullException(nameof(umbracoContextFactory));
    }

    public void Initialize()
    {
        if (_examineManager.TryGetIndex(global::Umbraco.Cms.Core.Constants.UmbracoIndexes.ExternalIndexName, out var externalIndex))
        {
            ((BaseIndexProvider)externalIndex).TransformingIndexValues += IndexerComponent_TransformingIndexValues;
        }
    }

    private void IndexerComponent_TransformingIndexValues(object? sender, IndexingItemEventArgs e)
    {
        if (int.TryParse(e.ValueSet.Id, out var nodeId))
        {
            switch (e.ValueSet.ItemType)
            {
                case "blogPage":
                    using (var umbracoContext = _umbracoContextFactory.EnsureUmbracoContext())
                    {
                        var contentNode = umbracoContext.UmbracoContext.Content?.GetById(nodeId);
                        if (contentNode != null)
                        {
                            var categories = contentNode.Value<IEnumerable<IPublishedContent>>("blogCategories");
                            var publishedContents = categories as IPublishedContent[] ?? categories?.ToArray();
                            var updatedValues = e.ValueSet.Values.ToDictionary(x => x.Key, x => x.Value.ToList());
                            if (publishedContents != null && publishedContents.Any())
                            {
                                if (updatedValues.ContainsKey(Constants.ExamineFields.SearchableBlogCategories))
                                {
                                    updatedValues[Constants.ExamineFields.SearchableBlogCategories] = new List<object> { string.Join(" ", publishedContents.Select(x => x.Key.ToString("N"))) };
                                }
                                else
                                {
                                    updatedValues.Add(Constants.ExamineFields.SearchableBlogCategories,new List<object> { string.Join(" ", publishedContents.Select(x => x.Key.ToString("N"))) });   
                                }
                            }
                            e.SetValues(updatedValues.ToDictionary(x => x.Key, x => (IEnumerable<object>)x.Value));
                        }
                    }
                    break;
            }
        }
    }

    public void Terminate()
    {
    }
}

This is all being hit, and the value added to the updateValues dictionary when I hit save. But, when I hit save again, those values are not in the index?

Any idea where I am going wrong with this?