asp.net-mvc-4 - Fluent Nhibernate と Web API エラー

Question

エラーがあります。以下のコードと例外メッセージを確認できます。

ユーザークラス

public class User 
    {
        public virtual int ID { get; set; }
        public virtual string UserName { get; set; }
        public virtual string PassWord { get; set; }
        public virtual DateTime AddedDate { get; set; }          
        public virtual UserGroup UserGroup { get; set; }

    }

費用収入のクラス

public class expenseincomes
    {
        public virtual int ID { get; set; }
        public virtual int ExpIncTypeID { get; set; }
        public virtual decimal Amount { get; set; }
        public virtual User User { get; set; }

    }

クラスのマップ

public class UserMap : ClassMap<User>
    {
       public UserMap() {
           Table("TUsers");
           Id(x => x.ID);
           Map(x => x.AddedDate);              
           Map(x => x.PassWord);
           Map(x => x.UserName);

       }

    }

  public class expenseincomesMap : ClassMap<expenseincomes>
    {
        public expenseincomesMap()
        {
           Table("texpenseincomes");
           Id(x => x.ID);
           Map(x => x.Amount);
           Map(x=>x.ExpIncTypeID);
           References(x => x.User).Column("UserID").Cascade.All();
       }
    }

Web API 構成クラス

config.EnableQuerySupport();
                    config.Routes.MapHttpRoute(
                    name: "ActionApi",
                    routeTemplate: "odata/{controller}/{action}/{id}",
                    defaults: new { id = RouteParameter.Optional }
                    );

                    var json = config.Formatters.JsonFormatter;
                    json.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;
                    config.Formatters.Remove(config.Formatters.XmlFormatter);

[http://localhost:2711/odata/expenseincomes/get] URL を呼び出すと、この例外メッセージが表示されます。

"$id":"1","Message":"An error has occurred.","ExceptionMessage":"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.","ExceptionType":"System.InvalidOperationException","StackTrace":null,"InnerException":{"$id":"2","Message":"An error has occurred.","ExceptionMessage":"Cannot serialize a Session while connected","ExceptionType":"System.InvalidOperationException","StackTrace":"   at NHibernate.Impl.SessionImpl.System.Runtime.Serialization.ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeISerializable(JsonWriter writer, ISerializable value, JsonISerializableContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeISerializable(JsonWriter writer, ISerializable value, JsonISerializableContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IWrappedCollection values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value)\r\n   at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value)\r\n   at Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)\r\n   at System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.<WriteToStreamAsync>b__c()\r\n   at System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token)"}}

しかし、URL => [http://localhost:2711/odata/Users/get] に問題はありません。