实体框架：将现有的POCO添加到新的父POCO中，在DB中创建新的子节点 [英] Entity Framework: adding existing child POCO to new Parent POCO, creates new child in DB
问题描述
当保存新学生时，下面的代码不合需要地插入一个新的课程记录记录，而我想要现有的课程记录链接到新的学生记录。
如何在Entity Framework中执行此操作...
- 对象可以与上下文断开连接。 （即在一个上下文中查询，然后保存在不同的上下文中）
- 我不需要从DB重新查询子记录，所以我可以将其附加到父项， m保存到db。当我已经将其作为内存中的对象，我真的想避免对数据库进行额外的访问。
此页面显示了一个数据库图，其代码基于 http://entityframeworktutorial.net/EF4_EnvSetup.aspx#.UPMZ4m-UN9Y
class Program
{
static void Main（string [] args）
{
//从db断开现有课程对象
var course = Program.getCourse（）;
//创建新学生
var stud = new Student（）;
stud.StudentName =bob;
//为学生分配现有的课程
stud.Courses.Add（course）;
//将学生保存到db
使用（SchoolDBEntities ctx = new SchoolDBEntities（））
{
ctx.Students.AddObject（stud）;
ctx.SaveChanges（）;
}
}
static课程getCourse（）
{
课程returnCourse = null;
使用（var ctx = new SchoolDBEntities（））
{
ctx.ContextOptions.LazyLoadingEnabled = false;
returnCourse =（from ct in ctx.Courses
select s）.SingleOrDefault（）;
}
return returnCourse;
}
}
相信没有办法实现这一点。
您可以指定该课程实体不变，而不是按照以下原则添加：
ctx.Entry（course） .State = EntityState.Unchanged;
或指示您的上下文，您正在使用现有实体：
ctx.Courses.Attach（course）;
更多信息：
http://msdn.microsoft.com/en-us/data/jj592676.aspx
编辑
我的解决方案中有一些运行的示例，我验证他们按预期工作。
在所有情况下，我们在ID = 2和Name =Addison Wesley的数据库中都有Publisher记录（与示例无关，但仅适用于较好的度量）。
方法1 - 设置实体状态
using（var context = new Context（））
{
var book = new Book（）;
book.Name =服务设计模式;
book.Publisher = new Publisher（）{Id = 2}; //只需要ID
context.Entry（book.Publisher）.State = EntityState.Unchanged;
context.Books.Add（book）;
context.SaveChanges（）;
}
方法2 - 使用附加方法
using（var context = new Context（））
{
var book = new Book（）;
book.Name =服务设计模式;
book.Publisher = new Publisher（）{Id = 2}; //只需要ID
context.Publishers.Attach（book.Publisher）;
context.Books.Add（book）;
context.SaveChanges（）;
}
方法3 - 设置外键值 （var context = new Context（））
{
var book = new Book（）;
$ b
book.Name =服务设计模式;
book.PublisherId = 2;
context.Books.Add（book）;
context.SaveChanges（）;
}
对于最后一种工作方式，我需要添加额外的属性PublisherId，它有根据NavigationPropertyName +'Id约定命名为EF自动提取：
public int PublisherId {get; set ;}
public Publisher Publisher {get; set;}
我在这里使用EF5代码首先，但与POCO非常相似。
Im wanting to use Entity Framework POCO in a disconnected (from context) mode. In my scenario I'm creating a new Parent object and want to attach an existing child object to it and then save it to the db.
The code below undesirably inserts a new Course record when saving a new Student record, when instead I want the existing Course record linked to the new Student record.
How can I do this in Entity Framework where...
- the objects can be disconnected from the context. (i.e. Queried in one context and then saved in a different context)
- I dont need to re-query the child record from the DB just so I can attach it to the parent when I'm saving to db. I really want to avoid doing extra trips to the db when I already have it as an object in memory.
This page shows a database diagram that the code below is based on http://entityframeworktutorial.net/EF4_EnvSetup.aspx#.UPMZ4m-UN9Y
class Program
{
static void Main(string[] args)
{
//get existing course from db as disconnected object
var course = Program.getCourse();
//create new student
var stud = new Student();
stud.StudentName = "bob";
//assign existing course to the student
stud.Courses.Add(course);
//save student to db
using (SchoolDBEntities ctx = new SchoolDBEntities())
{
ctx.Students.AddObject(stud);
ctx.SaveChanges();
}
}
static Course getCourse()
{
Course returnCourse = null;
using (var ctx = new SchoolDBEntities())
{
ctx.ContextOptions.LazyLoadingEnabled = false;
returnCourse = (from s in ctx.Courses
select s).SingleOrDefault();
}
return returnCourse;
}
}
I believe there are few ways of accomplishing this. You can specify that course entity is unchanged rather than added, along these lines:
ctx.Entry(course).State = EntityState.Unchanged;
Or instruct your context, that you are working with existing entity:
ctx.Courses.Attach(course);
More info here: http://msdn.microsoft.com/en-us/data/jj592676.aspx
EDIT
There are some running samples from my solution, I verified they work as expected. In all cases we have Publisher record in database with ID = 2 and Name = "Addison Wesley" (irrelevant to the example, but just for good measure).
Approach 1 - Setting Entity State
using (var context = new Context())
{
var book = new Book();
book.Name = "Service Design Patterns";
book.Publisher = new Publisher() {Id = 2 }; // Only ID is required
context.Entry(book.Publisher).State = EntityState.Unchanged;
context.Books.Add(book);
context.SaveChanges();
}
Approach 2 - Using Attach method
using (var context = new Context())
{
var book = new Book();
book.Name = "Service Design Patterns";
book.Publisher = new Publisher() { Id = 2 }; // Only ID is required
context.Publishers.Attach(book.Publisher);
context.Books.Add(book);
context.SaveChanges();
}
Approach 3 - Setting Foreign Key value
using (var context = new Context())
{
var book = new Book();
book.Name = "Service Design Patterns";
book.PublisherId = 2;
context.Books.Add(book);
context.SaveChanges();
}
For this last approach to work I needed to add extra property PublisherId, it has to be named according to NavigationPropertyName + 'Id" convention to be picked up by EF auotmatically:
public int PublisherId { get; set; }
public Publisher Publisher { get; set; }
I am using here EF5 Code First, but it is very similar to POCO.
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