ADO.NET导航属性(ADO.NET Navigation Properties)

 我在ADO.NET上使用导航属性和继承时遇到了一些问题。 
 
 这是我的数据模型： 
 
 
 首先，一些词汇： 
 
 
 
 分类=类别 
 Formulario = Form 
 Campo = Field 
 Imagem =图像 
 Paragrafo =段落 
 Escolha =选择 
 Texto =文字 
 Resposta =答案 
 
 
 所以，我正在尝试在Form上创建一个自定义属性，返回它的答案数。 
 
 正常的方法（我认为）将是： 
 
public partial class Formulario
{
    public int Respostas
    {
        get
        {
            List<Item> itens = this.Itens.ToList();

            IEnumerable<Campo> campos = from m in itens where m as Campo != null select (Campo)m;
            int soma = campos.Sum(m => m.Respostas.Count);

            return soma;
        }
    }
}
 
 但它不起作用。 itens列表返回0个元素。 但是，当我这样做时，它返回它应该的4个项目： 
 
public partial class Formulario
{
    public int Respostas
    {
        get
        {
            FormulariosDB db = new FormulariosDB();

            List<Item> itens = db.Items.Where(m => m.Formulario.Id == this.Id).ToList();

            IEnumerable<Campo> campos = from m in itens where m as Campo != null select (Campo)m;
            int soma = campos.Sum(m => m.Respostas.Count);

            return soma;
        }
    }
}
 
 它仅在我实例化整个数据模型时才有效。 有谁知道为什么？ 
 
 PS：我正在使用.toList()方法，因此我可以使用所有Linq查询，而不仅仅是Linq2Entities允许我使用的 

I'm having a little problem using Navigation properties and inheritance on ADO.NET.
 
This is my Data Model:
 
 
First, some vocabulary:
 
 
 
Categoria = Category
 Formulario = Form
 Campo = Field
 Imagem = Image
 Paragrafo = Paragraph
 Escolha = Choice
 Texto = Text
 Resposta = Answer
 
 
So, I'm trying to create a custom property on a Form returning it's answers count.
 
The normal approach (i think) would be:
 
public partial class Formulario
{
    public int Respostas
    {
        get
        {
            List<Item> itens = this.Itens.ToList();

            IEnumerable<Campo> campos = from m in itens where m as Campo != null select (Campo)m;
            int soma = campos.Sum(m => m.Respostas.Count);

            return soma;
        }
    }
}
 
But it doesn't work. The itens list returns 0 elements. But when I do as below, it return the 4 items it should:
 
public partial class Formulario
{
    public int Respostas
    {
        get
        {
            FormulariosDB db = new FormulariosDB();

            List<Item> itens = db.Items.Where(m => m.Formulario.Id == this.Id).ToList();

            IEnumerable<Campo> campos = from m in itens where m as Campo != null select (Campo)m;
            int soma = campos.Sum(m => m.Respostas.Count);

            return soma;
        }
    }
}
 
It only works when I instanciate the entire data model. Does anyone knows why?
 
PS: I'm using the .toList() method so I can use all Linq queries, not only the ones Linq2Entities allows me to

原文：https://stackoverflow.com/questions/4215608