Umbraco v12 - Add Custom field to Examine Index

MyServiceComposer.cs

public class MyServiceComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        builder.AddNotificationHandler<UmbracoApplicationStartedNotification, UmbracoStartedNotificationHandler>();
    }
}

UmbracoStartedNotificationHandler.cs

public class UmbracoStartedNotificationHandler : INotificationHandler<UmbracoApplicationStartedNotification>
{
    private readonly IExamineManager _examineManager;
    private readonly IUmbracoContextFactory _umbracoContextFactory;

    public UmbracoStartedNotificationHandler(IExamineManager examineManager, IUmbracoContextFactory umbracoContextFactory)
    {
        _examineManager = examineManager;
        _umbracoContextFactory = umbracoContextFactory;
    }

    /// <summary>
    /// Handles the <see cref="UmbracoApplicationStartedNotification"/> event by adding an event handler for the
    /// TransformingIndexValues event of an index provider.
    /// </summary>
    /// <param name="notification">
    /// The <see cref="UmbracoApplicationStartedNotification"/> object that contains the event data.
    /// </param>
    public void Handle(UmbracoApplicationStartedNotification notification)
    {
        if (!_examineManager.TryGetIndex(UmbracoConstants.UmbracoIndexes.ExternalIndexName, out IIndex index))
            throw new InvalidOperationException(
                $"No index found with the name {UmbracoConstants.UmbracoIndexes.ExternalIndexName}");

        if (index is not BaseIndexProvider indexProvider)
            throw new InvalidOperationException(
                $"{UmbracoConstants.UmbracoIndexes.ExternalIndexName} does not inherit from {nameof(BaseIndexProvider)}");
        
        // Add an event handler for the TransformingIndexValues event
        indexProvider.TransformingIndexValues += IndexProviderOnTransformingIndexValues;
    }

    /// <summary>
    /// Event handler for the TransformingIndexValues event of an index provider.
    /// </summary>
    /// <param name="sender">The object that triggered the event.</param>
    /// <param name="e">An instance of the <see cref="IndexingItemEventArgs"/> class that contains the event data.</param>
    private void IndexProviderOnTransformingIndexValues(object? sender, IndexingItemEventArgs e)
    {
        // If we are not dealing with content we can leave here, there is nothing to do
        if (e.ValueSet.Category != IndexTypes.Content) return;

        if (!e.ValueSet.Values.TryGetValue("__Key", out var key) || !Guid.TryParse(key[0]?.ToString(), out var contentKey))
        {
            return;
        }

        using var contextReference = _umbracoContextFactory.EnsureUmbracoContext();
        
        // Get the published content
        var publishedContent = contextReference?.UmbracoContext?.Content?.GetById(contentKey);
            
        // No content item so exit here as nothing further to do
        if (publishedContent == null) return;
            
        // Get the key for the root node
        var siteKey = publishedContent.AncestorOrSelf(1).Key;

        // Create a new dictionary including the existing values and the new siteId field
        var values =
            e.ValueSet.Values.ToDictionary(kvp => kvp.Key, kvp => (IEnumerable<object>) kvp.Value);
        values["siteKey"] = new List<object> { siteKey };

        // Set the values with the addition of the siteKey
        e.SetValues(values);
    }