dinner.cs

public class Dinner
    {
        /// <summary>
        /// Id (in that exact case) is used by Raven.
        /// If on a Url of /dinners/edit/162 Nancy will bind the Id property to 162 so,
        /// we have to make sure that it starts with dinners/ so Raven can identify it properly
        /// </summary>
        private string id;
        public string Id
        {
            get { return id; }
            set
            {
                if (!value.StartsWith("dinners"))
                    value = "dinners/" + value;

                id = value;
            }
        }

        //[HiddenInput(DisplayValue = false)]
        [JsonIgnore]
        public int DinnerID
        {
            get
            {
                return int.Parse(Id.Substring(Id.LastIndexOf("/") + 1));
            }
        }

        [Required(ErrorMessage = "Title is required")]
        [StringLength(50, ErrorMessage = "Title may not be longer than 50 characters")]
        public string Title { get; set; }

        [Required(ErrorMessage = "Event Date is required")]
        [Display(Name = "Event Date")]
        public DateTime EventDate { get; set; }

        [Required(ErrorMessage = "Description is required")]
        [StringLength(256, ErrorMessage = "Description may not be longer than 256 characters")]
        [DataType(DataType.MultilineText)]
        public string Description { get; set; }

        [StringLength(20, ErrorMessage = "Hosted By name may not be longer than 20 characters")]
        [Display(Name = "Host's Name")]
        public string HostedBy { get; set; }

        [Required(ErrorMessage = "Contact info is required")]
        [StringLength(20, ErrorMessage = "Contact info may not be longer than 20 characters")]
        [Display(Name = "Contact Info")]
        public string ContactPhone { get; set; }

        [Required(ErrorMessage = "Address is required")]
        [StringLength(50, ErrorMessage = "Address may not be longer than 50 characters")]
        [Display(Name = "Address, City, State, ZIP")]
        public string Address { get; set; }

        [UIHint("CountryDropDown")]
        public string Country { get; set; }

        //[HiddenInput(DisplayValue = false)]
        public double Latitude { get; set; }

        //[HiddenInput(DisplayValue = false)]
        public double Longitude { get; set; }

        //[HiddenInput(DisplayValue = false)]
        public string HostedById { get; set; }

        public virtual ICollection<RSVP> RSVPs { get; set; }

        public bool IsHostedBy(string userName)
        {
            return String.Equals(HostedById ?? HostedBy, userName, StringComparison.Ordinal);
        }

        public bool IsUserRegistered(string userName)
        {
            return RSVPs.Any(r => r.AttendeeNameId == userName || (r.AttendeeNameId == null && r.AttendeeName == userName));
        }

        [UIHint("LocationDetail")]
        [JsonIgnore]
        public LocationDetail Location
        {
            get
            {
                return new LocationDetail() { Latitude = this.Latitude, Longitude = this.Longitude, Title = this.Title, Address = this.Address };
            }
            set
            {
                this.Latitude = value.Latitude;
                this.Longitude = value.Longitude;
                this.Title = value.Title;
                this.Address = value.Address;
            }
        }
    }

    public class LocationDetail
    {
        public double Latitude;
        public double Longitude;
        public string Title;
        public string Address;
    }

index.cs

public class MostPopularDinners : AbstractIndexCreationTask<Dinner>
    {
        public MostPopularDinners()
        {
            this.Map = dinners => from dinner in dinners
                                  select new
                                  {
                                      dinner.Id,
                                      dinner.EventDate,
                                      dinner.RSVPs.Count
                                  };

            

            this.TransformResults = (database, results) => from result in results
                                                           let dinner = database.Load<Dinner>(result.Id)
                                                           select new 
                                                           {
                                                               DinnerID = dinner.DinnerID,
                                                               EventDate = dinner.EventDate,
                                                               Latitude = dinner.Latitude,
                                                               Longitude = dinner.Longitude,
                                                               Title = dinner.Title,
                                                               Description = dinner.Description,
                                                               RSVPCount = dinner.RSVPs.Count,
                                                               Url = dinner.DinnerID.ToString()
                                                           };
            
            this.Sort(dinner => dinner.RSVPs.Count, Raven.Abstractions.Indexing.SortOptions.Int);
        }
    }

jsondinner.cs

 public class JsonDinner
    {
        public int DinnerID { get; set; }
        public DateTime EventDate { get; set; }
        public string Title { get; set; }
        public double Latitude { get; set; }
        public double Longitude { get; set; }
        public string Description { get; set; }
        public int RSVPCount { get; set; }
        public string Url { get; set; }
    }

query.cs

//ERROR :: The field 'RSVPs_Count' is not indexed, cannot sort on fields that are not indexed

  var jsonDinners = DocumentSession.Query<Dinner, MostPopularDinners>()
                        .Where(x => x.EventDate >= DateTime.Now.Date)
                        .Take((int)parameters.limit)
                        .OrderByDescending(x => x.RSVPs.Count)
                        .As<JsonDinner>();