EntityFramework,Navigation properties' Changes cannot be SAVE - asp.net

Question

//Hi,the following code snippet ,I want to Change the model and it's related entities according to the model's naviagation properties,But only affected the model scale properties.There is NO Exception in runtime.I don't know WHY ,please help me,Thanks a lot!
public bool Update(Container model)
{
var mExist=dbContext.Container.SingleOrDefault(m=>m.TransportDocumentId==model.TransportDocumen tId && m.ContainerNo==model.ContainerNo);
if (mExist != null)
{
if (mExist.Id != model.Id)
{
throw new Exception("The same entity has been exist!.");
}
dbContext.Entry(mExist).State = EntityState.Detached;
}
var mOld = dbContext.Container.SingleOrDefault(m => m.Id == model.Id);
string oldContainerNo = model.ContainerNo;
if (mOld != null)
{
oldContainerNo = mOld.ContainerNo;
dbContext.Entry(mOld).State = EntityState.Detached;
}
dbContext.Container.Attach(model);
var mGoods = model.TransportDocument.Goods;
if (model.ContainerNo != oldContainerNo)
{
foreach (var g in mGoods)
{
var gn = g.GoodsContainerNo.Where(n => n.ContainerNo == oldContainerNo);
foreach (var gNo in gn)
{
//WHY,here CHANGES cannot be save to datastore!!!
gNo.ContainerNo = model.ContainerNo;
dbContext.Entry(gNo).State = EntityState.Modified;
}
}
}
dbContext.Entry(model).State = EntityState.Modified;
return dbContext.SaveChanges() >= 0;
}