Sitecore: Automatically Save Related Items using Glass Mapper

CompositeRelationshipSavingTask.cs

public class CompositeRelationshipSavingTask : IObjectSavingTask
{
    private static readonly ConcurrentDictionary<Type, SitecoreFieldConfiguration[]> _compositeRelationships = new ConcurrentDictionary<Type, SitecoreFieldConfiguration[]>();

    private SitecoreFieldConfiguration[] GetCompositeRelationshipFields(ObjectSavingArgs args)
    {
        var type = args.SavingContext.Config.Type;
        return _compositeRelationships.GetOrAdd(type, x => CompositeRelationshipsValueFactory(args));
    }

    private SitecoreFieldConfiguration[] CompositeRelationshipsValueFactory(ObjectSavingArgs args)
    {
        return args.SavingContext.Config.Properties
            .Where(property => Attribute.IsDefined(property.PropertyInfo, typeof(SitecoreCompositeRelationshipAttribute), true))
            .OfType<SitecoreFieldConfiguration>()
            .ToArray();
    }
}