POCO EF嵌套类的WCF序列化

我有一个问题。我有一个WCF服务作为我的数据访问我的实体框架。我需要返回一些对象。这是我的第一个模型:

[DataContract(IsReference = true)]
public partial class Osoba
{
    [DataMember]
    public int IdOsobe { get; set; }
    [DataMember]
    public string PrezimeOsobe { get; set; }
    [DataMember]
    public string ImeOsobe { get; set; }
    [DataMember]
    public virtual Partner Partner { get; set; }
}

这是我的第二个模型:

[DataContract(IsReference = true)]
public partial class Partner
{
    public Partner()
    {
        this.Dokument = new HashSet<Dokument>();
    }
    [DataMember]
    public int IdPartnera { get; set; }
    [DataMember]
    public string OIB { get; set; }
    [DataMember]
    public int TipPartnera { get; set; }
    [DataMember]
    public Nullable<int> IdMjestaSjedista { get; set; }
    [DataMember]
    public string AdrSjedista { get; set; }
    [DataMember]
    public Nullable<int> IdMjestaIsporuke { get; set; }
    [DataMember]
    public string AdrIsporuke { get; set; }
    [DataMember]
    public virtual ICollection<Dokument> Dokument { get; set; }
    [DataMember]
    public virtual Mjesto Mjesto { get; set; }
    [DataMember]
    public virtual Mjesto Mjesto1 { get; set; }
    [DataMember]
    public virtual Osoba Osoba { get; set; }
    [DataMember]
    public virtual Tvrtka Tvrtka { get; set; }
}

当我的服务返回Osoba时，Partner引用对象没有序列化。Partner = null。我做错了什么?我尝试过ServiceKnownType属性，没有。