Fluent Api и Web Api

Я новичок в Entity Framework и Fluent Api. Я создал несколько моделей, создал базу данных. Я не уверен, но я думаю, что настроил Fluent Api. В базе пока данные остутсвуют. Проблема вот в чем. Когда я ввожу в браузер контроллер для Web Api появляется ошибка: например: ...../api/AirportAPI.

the 'objectcontent 1' type failed to serialize the response body for content type '**application/xml**

Когда я настроил в Global.asax, то ошибка не изменилось:

the 'objectcontent 1' type failed to serialize the response body for content type 'application/json

Кстати, это тестовое задание месяц назад я задавал вопрос

Когда добавил данные в базу тоже ничего не поменялось. Можете помочь где я допустил ошибку.

Airport.cs

            using System;
            using System.Collections.Generic;
            using System.Linq;
            using System.Web;
            namespace WebServiceForAirport.Models
            {
                public class Airport
                {
                    public int ID { get; set; }
                    public string Country { get; set; }
                    public string City { get; set; }
                    public string NameAirport { get; set; }
                    public string Airline { get; set; }
                    public string CodeAirport { get; set; }
                    public bool Active { get; set; }
                    public virtual Information information { get; set; }
                    public virtual ICollection<Departure_Arrival> DepArrList { get; set; }
                    public virtual ICollection<PitStop> PitStopList { get; set; }
                    public Airport()
                    {
                        DepArrList = new List<Departure_Arrival>();
                        PitStopList = new List<PitStop>();
                    }
                }
            }

Departure-Arrival.cs

            using System;
            using System.Collections.Generic;
            using System.Linq;
            using System.Web;
            namespace WebServiceForAirport.Models
            {
                public class Departure_Arrival
                {
                    public int ID { get; set; }
                    public string Departure { get; set; }
                    public string Arrival { get; set; }
                    public int? AirID { get; set; }
                    public virtual Airport airport { get; set; }
                }
            }

Information.cs

        using System;
        using System.Collections.Generic;
        using System.ComponentModel.DataAnnotations;
        using System.ComponentModel.DataAnnotations.Schema;
        using System.Linq;
        using System.Web;
        namespace WebServiceForAirport.Models
        {
            /// <summary>
            /// 
            /// </summary>
            public class Information
            {
                public int ID { get; set; }
                public string NumberFlight { get; set; }
                public string NumberBoard { get; set; }
                public virtual Airport airport { get; set; }
            }
        }

PitStop.cs

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    namespace WebServiceForAirport.Models
    {
        /// <summary>
        /// 
        /// </summary>
        public class PitStop
        {
            public int ID { get; set; }
            public string PitStopCity { get; set; }
            public DateTime PitStopTime { get; set; }
            public int? AirID { get; set; }
            public virtual Airport airport { get; set; }
        }
    }

AirportDBContext.cs

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    using System.Data.Entity;

    namespace WebServiceForAirport.Models
    {
        public class AirDBContext:DbContext
        {
            public DbSet<Airport> Airports { get; set; }
            public DbSet<Departure_Arrival> Departires_Arrivals { get; set; }
            public DbSet<PitStop> PitStops { get; set; }
            public DbSet<Information> Informations { get; set; }

            protected override void OnModelCreating(DbModelBuilder modelBuilder)
            {
                //Primary Key for Airport
                modelBuilder.Entity<Airport>()
                    .HasKey(a => a.ID);
                //Primary Key for Departure-Arrival
                modelBuilder.Entity<Departure_Arrival>()
                    .HasKey(da => da.ID);
                //Primary Key for PitStop
                modelBuilder.Entity<PitStop>()
                    .HasKey(p => p.ID);
                //Primary Key for Information
                modelBuilder.Entity<Information>()
                    .HasKey(i => i.ID);

                //one-to-one Airport - Information
                modelBuilder.Entity<Airport>()
                    .HasRequired(a=>a.information)
                    .WithRequiredPrincipal(a=>a.airport);
                //one-to-many Airport - Departure-Arrival
                modelBuilder.Entity<Airport>()
                    .HasMany(a => a.DepArrList)
                    .WithRequired(da => da.airport);
                //one-to-many Airport - PitStop
                modelBuilder.Entity<Airport>()
                    .HasMany(a => a.PitStopList)
                    .WithRequired(pi => pi.airport);
                base.OnModelCreating(modelBuilder);
            }
        }
    }

Global.asax.cs

    public class Global : HttpApplication
    {
        void Application_Start(object sender, EventArgs e)
        {
            // Code that runs on application startup
            AreaRegistration.RegisterAllAreas();
            GlobalConfiguration.Configure(WebApiConfig.Register);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings
                .ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
            GlobalConfiguration.Configuration.Formatters.Remove(GlobalConfiguration.Configuration.Formatters.XmlFormatter);
        }
    }

Web.Config

  <connectionStrings>
    <add name="AirDBContext" connectionString="Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename='|DataDirectory|\AirportDataBase.mdf';Integrated Security=True"
 providerName="System.Data.SqlClient"/>
  </connectionStrings>

AirportAPIController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using WebServiceForAirport.Models;
using System.Data.Entity;
namespace WebServiceForAirport.Controllers
{
    public class AirportAPIController : ApiController
    {
        AirDBContext db;
        public AirportAPIController()
        {
            db = new AirDBContext();
        }
        public IEnumerable<Airport> GetAll()
        {
            return db.Airports;
        }
    }
}