深入SQL Entity与EntityRef<T>内部

引言

最近在学习LINQ，已经逐步深入到LINQ to SQL和LINQ to Entity的部分。但是对SQL Entity专题中的EntityRef<T>很是迷惑，始终未能理解它是如何从数据库的1:1关系转变为C#中两个对象的相互引用的，于是有了这段摸索。

阅读本文，需要对SQL Entity的内部结构有一定程度的了解。

一切尽在不言中

线索

问题源自于下面我自己写的这段旨在测试引用关系的代码，Customers与Orders是通过“LINQ to SQL类”（.dbml）的方式，从数据库生成的SQL Entity类。其中，测试用例的客户Thomas　Hardy的CustomerID为AROUT。

using (NorthwindDataContext db = new NorthwindDataContext())
{
    db.Log = Console.Out;
    Customers customer = db.Customers.Where(c => c.ContactName == "Thomas Hardy").SingleOrDefault();
    Console.WriteLine("the customer {0} lives in {1}", customer.City);
    Console.WriteLine("he gets {0} orders.", customer.Orders.Count);

    Orders order = db.Orders.Where(o => o.CustomerID == "AROUT").FirstOrDefault();
    Console.WriteLine("get the first order id is: {0}", order.OrderID);

    Console.WriteLine(object.ReferenceEquals("is the reference equal? {0}", customer, order.Customers));
}

运行的结果

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[ContactName] = @p0
-- @p0: Input NVarChar (Size = 4000; Prec = 0; Scale = 0) [Thomas Hardy]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

the customer Thomas Hardy lives in London
SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [
t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[Sh
ipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPosta
lCode], [t0].[ShipCountry]
FROM [dbo].[Orders] AS [t0]
WHERE [t0].[CustomerID] = @p0
-- @p0: Input NVarChar (Size = 4000; Prec = 0; Scale = 0) [AROUT]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

he gets 13 orders.
SELECT TOP (1) [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[Order
Date], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight],
[t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[S
hipPostalCode], [t0].[ShipCountry]
FROM [dbo].[Orders] AS [t0]
WHERE [t0].[CustomerID] = @p0
-- @p0: Input NVarChar (Size = 4000; Prec = 0; Scale = 0) [AROUT]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

get the first order id is: 10355
is the reference equal? True