如何使用嵌套的opbjects使用DBContext.Add / Attach（使用EF CodeFirst 4.1） [英] How to use DBContext.Add/Attach (using EF CodeFirst 4.1) with nested opbjects

问题描述

问题：将对象Order添加到我的dbcontext中时，订单的所有嵌套对象都将被读取到数据库中，尽管嵌套对象是静态数据，但只有在数据库中添加了参考。 p>

示例：
数据库包含0个订单和3个项目。

我添加一个订单2项目。

现在数据库保存1个订单，5个项目。订单中的两个项目已经被读取到数据库，即使这些项目在db.SaveChanges（）之前具有正确的主键。

我意识到我可能可以在保存更改之前将现有项目附加到dbcontext，但那真的是唯一的办法吗？当主键与现有项目匹配时，EF不能确定项目已经存在？

有没有人知道新版本的EF CodeFirst是否有所不同？

解决方案

否

你必须确定正确图中每个实体的状态，例如使用：

  dbContext.Orders.Add（newOrder）; 
 foreach（newOrder.Items中的var item）{
 dbContext.Entry（item）.State = EntityState.Unchanged; 
} 
 dbContext.SaveChanges（）; 
  

您可以使用反向操作调用 Attach（newOrder）并将顺序设置为添加状态。主要区别在于独立关联 （例如多对多关系）。第一种方法将在订单和每个项目之间正确添加新关系，而第二种方法除非您手动将每个关系设置为添加状态（并且关系更改状态更复杂）。

Problem: When adding an object "Order" to my dbcontext, all nested objects of the order gets "readded" to the database, though the nested objects is static data and only a reference shoudl be added in the database.

Example: The database holds 0 orders, and 3 items.

I add one order with 2 items.

Now the database hold 1 order, and 5 items. The two items in the order has been "readded" to the database, even though the items had the right primary keys before db.SaveChanges().

I realize that i may be able to attach the existing items to the dbcontext before saving changes, but is that really the only way to go? Can't EF figure out that to item already exists when the primary key matches an existing item?

Does anyone know if this is different in the new version of EF CodeFirst?

解决方案

No EF cannot figure if entities are existing one or new one - both Add and Attach commands are graph oriented operations. You call them on one entity in the graph and they traverse all relations (and their relations and so on) and perform the operation for them as well.

You must figure correct state of each entity in the graph for example by using:

dbContext.Orders.Add(newOrder);
foreach(var item in newOrder.Items) {
    dbContext.Entry(item).State = EntityState.Unchanged;
}
dbContext.SaveChanges();

You can use the reverse operation by calling Attach(newOrder) and set the order to Added state. The main difference will come with independent associations (for example many-to-many relations). The first approach will correctly add new relation between order and each item whereas second will not unless you manually set each relation to Added state (and changing state for relations is more complex).

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