Question

我想知道是否可以连接添加到DbContext的新创建的（poco）实体（以便在导航时延迟加载子对象）

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

（注意：我知道我可以获取Location对象并直接分配它，上面的例子只是为了这个问题的目的）

（注意II：我可以创建一个代理对象，然后复制这些值，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？

Answer 1

简短的回答是，在使用外键和POCO时，您可以在设置FK之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，在概念级别而不是外键属性上工作对您来说更容易。

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。 documentation表示只有在对象具有永久密钥时才会出现这种情况。

如果引用处于添加状态（在此示例中为课程对象），引用导航属性将不会同步使用新对象的键值，直到调用SaveChanges。不会发生同步，因为对象上下文不会包含添加对象的永久键，直到它们被保存。

但是，当处理Added状态且没有永久密钥的新添加实体的属性时，文档似乎不正确或具有误导性，并且仍然会发生导航属性和外键的同步。 / p>

以下是我自己的数据模型上的一些测试代码，我用它来研究这个问题。这是使用EF5和.Net4，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");

实体框架 - 连接新添加的Poco实体并加载子对象（插入/添加）

1 个答案: