具有依赖项的Linq to SQL深度复制对象

我正在尝试创建一个linq-to-sql对象的列表,稍后我将添加到DataContext并插入该列表。当我调用SubmitChanges()时,我得到一个错误,说明WeatherForecast对象的Postcode外键为空。

当调用list.add()时,它不会对依赖对象进行深度复制。有没有办法让它做一个深度复制?

这是我想做的一个例子。

function List<WeatherForecast> CreateForecast(Postcode postcode) 
{ 
   var forecasts = List<WeatherForecast>();

   var wf = new WeatherForecast() 
   { 
      ForecastDate = DateTime.Today
      , CurrentTemperature = (decimal)today.Attribute(XName.Get("temperature"))
   };
   wf.Postcode = postcode;

   forecasts.Add(wf); 

   ...
   Keep adding forecast objects into the list
   ...

   return forecasts;
}