无法更新实体中的外键

These are my models:

这些是我的模特:

public class Post
{
    [Key]
    public int Id { get; set; }

    public string Title { get; set; }

    [DefaultValue(null)]
    public virtual Image Thumbnail { get; set; }

    public void SetViewModelValues(CreatePostViewModel viewModel)
    {
        Title = viewModel.Title;
    }
}

public class CreatePostViewModel
{
    public string Title { get; set; }

    public int? ThumbnailId { get; set; }
}
public class