AuthorRelations.cs

public class Handler(IUmbracoContextFactory umbracoContextFactory, IRelationService relationService) : INotificationHandler<ContentPublishedNotification>
{
    public void Handle(ContentPublishedNotification notification)
    {
        using var umbracoContext = umbracoContextFactory.EnsureUmbracoContext().UmbracoContext;
        foreach (var content in notification.PublishedEntities.Where(x => x.ContentType.Alias == Article.ModelTypeAlias))
        {
            if (umbracoContext.Content?.GetById(content.Id) is Article article)
            {
                relationService.Relate(article.Author.Id, article.Id, "authorToArticle");
            }
        }
    }
}