EF: How to read complex object from API POST?

Question

I have the following classes:

public partial class ReservationVersion
{
    public ReservationVersion()
    {
        this.Reservations = new HashSet<Reservation>();
        this.ReservationItems = new HashSet<ReservationItem>();
    }

    public int ReservationVersionID { get; set; }
    public Nullable<int> ReservationID { get; set; }
    public string GUID { get; set; }
    public string StartDate { get; set; }
    public string StartTime { get; set; }
    public string Duration { get; set; }    

    public virtual ICollection<Reservation> Reservations { get; set; }
    public virtual Reservation Reservation { get; set; }
    public virtual ICollection<ReservationItem> ReservationItems { get; set; }

}


public abstract partial class ReservationItem
{
    public ReservationItem()
    {
    }

    public int ReservationItemID { get; set; }
    public Nullable<int> ReservationVersionID { get; set; }
    public string GUID { get; set; }
    public Nullable<int> ParentID { get; set; }
    public Nullable<bool> NeedsConfirmation { get; set; }
    public string ReservationSummary { get; set; }
    public string Comment { get; set; }
    public Nullable<int> ObjectStateID { get; set; }
    public string StartDate { get; set; }
    public string StartTime { get; set; }
    public string Duration { get; set; }

    public virtual ReservationVersion ReservationVersion { get; set; }
}

public abstract partial class Appointment : ReservationItem
{
    public Appointment()
    {
    }

    public string AppointmentDescription { get; set; }
}

public partial class AppointedActivity : Appointment
{
    public Nullable<int> AppointedActivityID { get; set; }

    public virtual Activity Activity { get; set; }
}

public partial class AppointedDevice : Appointment
{
    public Nullable<int> AppointedDeviceID { get; set; }

    public virtual Device Device { get; set; }
}

I'm using Entity Framework and I'm having trouble understanding why I can't correctly parse a complex object from a JSON post.

Here's the JSON being sent (which is a valid ReservationVersion):

{
    "ObjectStateID": 1,
    "StartDate": "2017-10-31",
    //Other props
    "ReservationItems": [
      {
        "ReservationSummary": "Act",
        "AppointedActivityID": 95874,
        "AppointmentDescription": "ACT"
      },
      {
        "ReservationSummary": "Device",
        "AppointedDeviceID": 99040,
        "AppointmentDescription": "DEV"
      }
    ]
}

I can correctly read the ReservationVersion but the collection of ReservationItems is empty every time.

My guess is that there's a problem with ReservationItems being an abstract class and the items I'm sending should correspond to AppointedSomething, which inherits from Appointment, which inherits from ReservationItems

Any help would be much appreciated