实体框架 - 断开的上下文和重复的记录答案

【问题标题】：Entity Framework - Disconnected Contexts and Duplicate Records实体框架 - 断开的上下文和重复的记录
【发布时间】：2017-06-09 15:47:55
【问题描述】：

我正在使用 Entity Framework 6。当我将导航属性更新为一个对象，然后执行 SaveChanges()，而不是引用现有的对象时，它会在引用表中创建一个副本。

下面有一些例子：

以下示例按预期工作。它创建了一个“客户”和一个“订单”并成功地将它们关联起来：

// Homer Simpson buys some Duff beer

Console.WriteLine("Homer Simpson is buying Duff beer");

Customer customer = new Customer
{
    Name = "Homer Simpson"
};

Order order1 = new Order
{
    Description = "Duff Beer"
};

customer.Orders.Add(order1);

using (EFContext db = new EFContext())
{
    db.Customers.Add(customer);
    DisplayChanges(db);
    db.SaveChanges();
}

输出

Homer Simpson is buying Duff beer
ConsoleApplication1.Objects.Customer - Added
ConsoleApplication1.Objects.Order - Added

但是，如果我尝试添加订单并关联现有客户，当我保存更改时，它会复制数据库中的客户对象以及我创建的现有订单：

// Homer Simpson buys some Duff beer

Console.WriteLine("Homer Simpson is buying Duff beer");

Customer customer = new Customer
{
    Name = "Homer Simpson"
};

Order order1 = new Order
{
    Description = "Duff Beer"
};

customer.Orders.Add(order1);

using (EFContext db = new EFContext())
{
    db.Customers.Add(customer);
    DisplayChanges(db);
    db.SaveChanges();
}

// Homer Simpson buys some Buzz Cola

Console.WriteLine("\r\nHomer Simpson is buying Buzz Cola");

Order order2 = new Order
{
    Description = "Buzz Cola",
    Customer = customer
};

using (EFContext db = new EFContext())
{
    db.Orders.Add(order2);
    DisplayChanges(db);
    db.SaveChanges();
}

输出

Homer Simpson is buying Duff beer
ConsoleApplication1.Objects.Customer - Added
ConsoleApplication1.Objects.Order - Added

Homer Simpson is buying Buzz Cola
ConsoleApplication1.Objects.Order - Added
ConsoleApplication1.Objects.Customer - Added
ConsoleApplication1.Objects.Order - Added

如果我设置外键而不是导航属性，则记录不会重复。但是，在这种情况下，我无法访问相关对象的属性，直到我明确加载它：

// Homer Simpson buys some Duff beer

Console.WriteLine("Homer Simpson is buying Duff beer");

Customer customer = new Customer
{
    Name = "Homer Simpson"
};

Order order1 = new Order
{
    Description = "Duff Beer"
};

customer.Orders.Add(order1);

using (EFContext db = new EFContext())
{
    db.Customers.Add(customer);
    DisplayChanges(db);
    db.SaveChanges();
}

// Homer Simpson buys some donuts

Console.WriteLine("\r\nHomer Simpson is buying donuts");

Order order3 = new Order
{
    Description = "Donuts",
    CustomerID = 1
};

if (order3.Customer == null)
{
    Console.WriteLine("Customer is null before save!");
}

else
{
    Console.WriteLine("The customer is " + customer.Name + " before save!");
}

using (EFContext db = new EFContext())
{
    db.Orders.Add(order3);
    DisplayChanges(db);
    db.SaveChanges();
}

if (order3.Customer == null)
{
    Console.WriteLine("Customer is null after save!");
}

else
{
    Console.WriteLine("The customer is " + customer.Name + " after save!");
}

using (EFContext db = new EFContext())
{
    db.Orders.Attach(order3);
    db.Entry(order3).Reference(x => x.Customer).Load();
}

if (order3.Customer == null)
{
    Console.WriteLine("Customer is null after forced load!");
}

else
{
    Console.WriteLine("The customer is " + customer.Name + " after forced load!");
}

输出

Homer Simpson is buying Duff beer
ConsoleApplication1.Objects.Customer - Added
ConsoleApplication1.Objects.Order - Added

Homer Simpson is buying donuts
Customer is null before save!
ConsoleApplication1.Objects.Order - Added
Customer is null after save!
The customer is Homer Simpson after forced load!

有人可以提供任何建议吗？我知道您可以手动附加和分离实体，但话虽如此，我正在处理一些非常复杂的对象。我想找到一种简单的方法来控制实体框架中的更改跟踪。

问候

彼得·迪根

【问题讨论】：

没有简单的方法 - 在 EF 中使用断开连接的对象图从来都不是一件容易/简单的事情。您可以尝试尝试解决该主题的GraphDiff 包。不幸的是，作者不再支持它。

标签： entity-framework entity-framework-6

【解决方案1】：

当您将客户对象附加到订单 2 时，它正在插入一个新客户对象，我认为您应该始终使用外键而不是附加实体，并加载属性部分当然，您不会将客户对象附加到订单，因为您只是插入了它如果您检查public virtual <T> Add(T entity)，因为您可以看到它返回对象本身，而不包括任何其他对象，客户对象在您加载它之前不会被加载，我猜这是有道理的，在您的示例中，您使用显式加载来加载客户对象，如果您想避免这种情况，您可以启用延迟加载并始终获取客户对象而不显式加载它。简而言之，始终使用 fk，并且当您需要实体加载它（延迟加载或显式）时，附加对象绝不是好的做法，除非您想将所有对象插入在一起，或者您不关心重复项。我希望这会有所帮助。

【讨论】：