为什么在我的实体框架中添加一个子元素会尝试插入一个新的父元素

我在WCF中有一个C#REST服务，它位于SQL 2008上的EF 4.2框架之上。有几个实体，但重要的两个是Trips和他们的孩子PlacesOfInterest。

以下是表格：

-- Creating table 'Trips'
CREATE TABLE [dbo].[Trips] (
[Id] uniqueidentifier  NOT NULL,
[Name] nvarchar(max)  NOT NULL,
[ArrivalDate] datetime  NULL,
[DepartureDate] datetime  NULL,
[WhereTo] nvarchar(max)  NOT NULL,
[CreatedDate] datetime  NULL,
[UpdatedDate] datetime  NULL,
[Album] nvarchar(max)  NOT NULL,
[UserToken] nvarchar(max)  NOT NULL,
[WallPostId] nvarchar(max)  NOT NULL
);
GO
-- Creating table 'PlacesOfInterest'
CREATE TABLE [dbo].[PlacesOfInterest] (
[Id] uniqueidentifier  NOT NULL,
[PhoneNumber] nvarchar(max)  NOT NULL,
[SmallPicture] nvarchar(max)  NOT NULL,
[LargePicture] nvarchar(max)  NOT NULL,
[HostId] nvarchar(max)  NOT NULL,
[HostName] nvarchar(max)  NOT NULL,
[Address1] nvarchar(max)  NOT NULL,
[Address2] nvarchar(max)  NOT NULL,
[Address3] nvarchar(max)  NOT NULL,
[City] nvarchar(max)  NOT NULL,
[State] nvarchar(max)  NOT NULL,
[Zip] nvarchar(max)  NOT NULL,
[Latitude] bigint  NOT NULL,
[Longitude] bigint  NOT NULL,
[URL] nvarchar(max)  NOT NULL,
[MapUrl] nvarchar(max)  NOT NULL,
[Hours] nvarchar(max)  NOT NULL,
[Name] nvarchar(max)  NOT NULL,
[PageId] nvarchar(max)  NOT NULL,
[Trip_Id] uniqueidentifier  NOT NULL,
[Category_Id] int  NULL
);
GO

这是我的POCO模型。

public class Trip
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public DateTime ArrivalDate { get; set; }
    public DateTime DepartureDate { get; set; }
    public string WhereTo { get; set; }
    public DateTime CreatedDate { get; set; }
    public DateTime UpdatedDate { get; set; }
    public string Album { get; set; }
    public List<PlaceOfInterest> PlacesOfInterest { get; set; }
    public List<Photo> Photos { get; set; }
    public string UserToken { get; set; }
    public string WallPostId { get; set; }
}
public class PlaceOfInterest
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public string PhoneNumber { get; set; }
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string Address3 { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Zip { get; set; }
    public long Latitude { get; set; }
    public long Longitude { get; set; }
    public string SmallPicture { get; set; }
    public string LargePicture { get; set; }
    public string HostId { get; set; }
    public string HostName { get; set; }
    public string URL { get; set; }
    public string MapUrl { get; set; }
    public string Hours { get; set; }
    public Category Category { get; set; }
    public List<Photo> Photos { get; set; }
    public List<Checkin> Checkins { get; set; }
    public List<PoiAttribute> PoiAttributes { get; set; }
    public Guid TripId { get; set; }
    public string PageId { get; set; }
    [IgnoreDataMember]
    public Trip Trip { get; set; }
}

当我添加一个新的PlaceOfInterest时，实体框架试图插入一个列表，但我得到了一个重复的键错误。

    public static Guid Create(string tripId, Model.PlaceOfInterest instance)
    {
        var context = new Model.POCOTripContext();
        var cleanPoi = new Model.PlaceOfInterest();
        cleanPoi.Id = Guid.NewGuid();
        cleanPoi.Address1 = instance.Address1;
        cleanPoi.Address2 = instance.Address2;
        cleanPoi.Address3 = instance.Address3;
        cleanPoi.City = instance.City;
        cleanPoi.HostId = instance.HostId;
        cleanPoi.HostName = instance.HostName;
        cleanPoi.Hours = instance.Hours;
        cleanPoi.LargePicture = instance.LargePicture;
        cleanPoi.Latitude = instance.Latitude;
        cleanPoi.Longitude = instance.Longitude;
        cleanPoi.MapUrl = instance.MapUrl;
        cleanPoi.Name = instance.Name;
        cleanPoi.PageId = instance.PageId;
        cleanPoi.PhoneNumber = instance.PhoneNumber;
        cleanPoi.SmallPicture = instance.PhoneNumber;
        cleanPoi.State = instance.State;
        cleanPoi.Trip = Library.Trip.Get(new Guid(tripId));
        cleanPoi.URL = instance.URL;
        cleanPoi.Zip = instance.Zip;
        context.PlacesOfInterest.AddObject(cleanPoi);
        context.SaveChanges();
        return cleanPoi.Id;
    }

我不希望它插入那个行程，我只希望它更新数据库中的TripId列。我做错了什么？