实体框架-连接新添加的Poco实体并加载子对象(插入/添加)

Question

我想知道是否有可能连接添加到DbContext的新创建的（poco）实体（这样子对象就可以在导航中延迟加载）

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

（注意：我知道我可以获得Location对象并直接分配它，上面的例子只是为了解决这个问题）

（注意II：我可以创建一个代理对象，然后复制值，这样的对象会被连接（并进行延迟加载），但任何复杂的子对象（其他pocos）都不会包括在复制中）

有没有办法连接（或加载）添加的poco？

Answer 1

简单的答案是，当使用外键和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，那么值得考虑的是，在概念级别而不是使用外键属性工作是否更容易。

当上下文跟踪导航属性表示的实体时，可以在DetectChanges（）上同步FK和nav属性。文档表明，只有当对象具有永久密钥时，才会发生这种情况。

如果引用处于添加状态（在本例中，课程对象），则不会同步引用导航属性使用新对象的键值，直到调用SaveChanges为止。由于对象上下文没有包含已添加对象的永久关键帧，直到保存为止。

然而，文档似乎是不正确或误导性的，当使用处于Added状态且没有永久密钥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。

下面是我自己的数据模型上的一些测试代码，我用来研究这个问题。这是将EF5与.Net 4一起使用，据我所知，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");