LINQ To SQL Include existing object in insert

In this method, I am inserting a new item (Room) into the database. That process functions as expected.

But, in addition to that, each time I add a room, I want to add a piece of furniture as the initial piece. Each item of type Furniture has a "RoomID" to designate its location. Thus, Room contains a collection of Furniture. Below, I am the piece of "primary" furniture from the database, adding it to the room's furniture collection, and submitting changes. The room gets added to the database, but the Furniture.RoomID column remains as null.

public void AddResidentToUniverse(int residentID, int universeID)
{
    Universe uni = _context.Universes.FirstOrDefault(u => u.UniverseID == universeID);
    Resident res = _context.Residents.FirstOrDefault(r=>r.ResidentID == residentID);

    if (uni != null && res!=null)
    {
        Room e = new Room();
        Furniture primary = _context.Furnitures.FirstOrDefault(p => p.FurnitureID == new FurnitureController().GetPrimary(universeID).FurnitureID);

        e.UniverseID = uni.UniverseID;
        e.RoomName = res.RootName;
        e.ResidentID = residentID;
        e.Expired = null;
        e.Furniture.Add(primary);
        uni.Rooms.Add(e);

        _context.SubmitChanges();
    }
}