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我想知道是否可以连接新创建的（poco）实体添加到 DbContext（以便子对象可以在导航时延迟加载）

var user = new User();
user.LocationID = 3;
dbContext.Users.Add(user);
var locationName = user.Location.Name;

（注：我明白我可以直接获取Location对象并赋值，上面的例子只是为了这个问题的目的）

（注二：我可以创建一个代理对象然后复制值，这样的对象将被连接（并进行延迟加载）但任何复杂的子对象（其他 pocos）都不会包含在副本中）

有没有办法连接（或加载）添加的 poco？

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1 回答 1

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简短的回答是，在使用外键和 POCO 时，您可以在设置 FK 后显式延迟加载引用属性，如下所示：

context.Entry(e).Reference(propName).Load();

如果您需要经常遍历对象图，那么值得考虑在概念级别上工作而不是使用外键属性对您来说更容易。

当上下文跟踪由导航属性表示的实体时，可以在 DetectChanges() 上同步 FK 和导航属性。文档表明，这只发生在对象具有永久密钥时。

如果引用处于添加状态（在此示例中为课程对象），则在调用 SaveChanges 之前，引用导航属性不会与新对象的键值同步。不会发生同步，因为对象上下文不包含添加对象的永久键，直到它们被保存。

但是，文档似乎不正确或具有误导性，并且在使用处于Added状态且没有永久键的新添加实体的属性时，导航属性和外键的同步仍然会发生。

下面是我用来研究这个的我自己的数据模型上的一些测试代码。这是将 EF5 与 .Net4 一起使用，我理解它实际上是 EF4.3。

using (MyContext context = new MyContext())
{
  /*
   * Uncomment these lines to test assigning FK Id when entity is being tracked
   */
  Customer cust1 = context.Customers.OrderBy(x => x.Id).First();
  Customer cust2 = context.Customers.OrderBy(x => x.Id).Skip(1).First();

  /*
   * Uncomment these lines to test assigning FK Id 
   * without having entity loaded in ObjectStateManager
   */
  //Customer cust1 = context.Customers.AsNoTracking().OrderBy(x => x.Id).First();
  //Customer cust2 = context.Customers.AsNoTracking()
  //                         .OrderBy(x => x.Id).Skip(1).First();

  //new entities
  Quote proxyQ = context.Quotes.Create();
  Quote pocoQ = new Quote();

  /*
   * if adding the new entities to context before setting FK properties
   * DetectChanges must be called later to attempt sync with nav props
   */
  context.Quotes.Add(proxyQ);
  context.Quotes.Add(pocoQ);

  //set FK Customer ids
  proxyQ.CustomerId = cust1.Id;
  pocoQ.CustomerId = cust2.Id;

  /*
   * FK / nav prop sync happens on DetectChanges() if the Customer 
   * entity is being tracked
   * it must be explicitly called if it has not been called using 
   * one of the AutoDetectChanges functions in order to sync
   */
  context.ChangeTracker.DetectChanges();

  /*
   * Alternatively, if the new entities are added to context after setting FK props
   * and AutoDetectChanges is enabled then DetectChanges is called implicitly and
   * FK / nav prop sync will happen here if the matching Customer entity is 
   * being tracked by the context.
   */
  //context.Quotes.Add(proxyQ); 
  //context.Quotes.Add(pocoQ);

  /*
   * If assigning FK Id and the entity the id represents is not tracked then 
   * proxy will lazy load Customer here.
   */
  Console.WriteLine("Proxy quote with key {0} linked to customer with name {1}",
   proxyQ.Id.ToString(), proxyQ.Customer != null ? proxyQ.Customer.Name : "null");

  /*
   * Obviously no lazy loading of Customer can occur here for a POCO
   */
  Console.WriteLine("POCO quote with key {0} linked to customer with name {1}",
   pocoQ.Id.ToString(), pocoQ.Customer != null ? pocoQ.Customer.Name : "null");

  /*
   * But we can explicit lazy load when using POCO if we have assigned 
   * an FK Customer Id to a POCO quote and the Customer entity is not 
   * being tracked by context
   */
   //context.Entry(pocoQ).Reference("Customer").Load();
   //Console.WriteLine("POCO quote with key {0} linked to customer with name {1}",
   //  pocoQ.Id.ToString(), pocoQ.Customer != null ? pocoQ.Customer.Name : "null");
于 2013-06-29T05:55:29.200 回答