buechermarkt/BuechermarktClient/Models/Book.cs

using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Bson;
using System;

namespace BuechermarktClient.Models
{
   public class Book
   {
      [BsonId]
      public ObjectId ID { get; set; }
      
      [BsonElement("student")]
      public ObjectId Student { get; set; }
      
      [BsonElement("book_type")]
      public ObjectId BookType { get; set; }

      [BsonElement("price")]
      public double Price { get; set; }

      [BsonElement("state")]
      public BookState State { get; set; }

      [BsonElement("label_id")]
      public string LabelId { get; set; }
   }

   public enum BookState
   {
      InStock,
      Selled,
      BackToStudent,
      Missing
   }
}