deeply nested shitstorm ;-)

Program.cs

using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Entities;
using MongoDB.Entities.Core;
using System.Linq;

namespace TestApp
{
    public class Team : Entity
    {
        public Dashboard[] Dashboards { get; set; }
    }

    public class Dashboard
    {
        public Version[] Versions { get; set; }
    }

    public class Version
    {
        public Widget[] Widgets { get; set; }
    }

    public class Widget
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string ID { get; set; }
        public string WidgetName { get; set; }
    }

    internal static class Program
    {
        private static void Main()
        {
            new DB("test");

            var widgetID = ObjectId.GenerateNewId().ToString();

            new Team
            {
                Dashboards = new[] {
                    new Dashboard {
                        Versions = new[] {
                            new Version {
                                Widgets = new[] {
                                    new Widget {
                                        ID = widgetID,
                                        WidgetName = "test widget" } } } } } }
            }.Save();

            var widget = DB.Fluent<Team>()
                           .Match(f => f.Eq("Dashboards.Versions.Widgets._id", ObjectId.Parse(widgetID)))
                           .Unwind("Dashboards")
                           .Unwind("Dashboards.Versions")
                           .Unwind("Dashboards.Versions.Widgets")
                           .ReplaceWith<Widget>("$Dashboards.Versions.Widgets")
                           .Match(w => w.ID == widgetID)
                           .SingleOrDefault();

            DB.Update<Team>()
              .Match(f => f.Eq("Dashboards.Versions.Widgets._id", ObjectId.Parse(widgetID)))
              .WithArrayFilter("{ 'x._id' : { $eq : ObjectId('" + widgetID + "') } }")
              .Modify("{ $set : { 'Dashboards.$[].Versions.$[].Widgets.$[x].WidgetName' : 'updated widget name' } }")
              .Execute();

        }
    }
}