实体框架：多对多关系，插入和更新 [英] Entity Framework : many to many relationship , Insert and update

问题描述

我有一个新闻实体，并且根据他们的NewsID得到消息。那么我定义了一个新的实体，一个组，我想根据他们的组ID获取消息。我尝试使用这篇文章通过使用代码第一种方法。

所以在我的上下文中我添加了：

  public class Groupnews：DbContext 
 {
 
 public DbSet< Group>组{get;组; } 
 public DbSet< News>新闻{get;组; 
 
 public Groupnews（）
：base（MyDb）
 {
} 
 
 public int NewsID {get;组; 
} 
 
 this.HasMany（t => t.News）
 .WithMany（t => t.Groups）
 .Map（m = &$ 
 {
 m.ToTable（GroupNews）; 
 m.MapLeftKey（GroupID）; 
 m.MapRightKey（NewsID）; 
 }）; 
  

现在我可以使用这种方法获取基于他们的GroupID的消息。但问题是插入新消息和更新。为此，我需要在GroupNews表中保存NewsID和GroupId。做这个在新闻模型中，我定义：

  public virtual ICollection< Group>相关组{get;组; } 
 
 public News（）
 {
 RelatedGroups = new List< Group>（）; 
} 
  

与组相同：

  public virtual ICollection< News>相关新闻组; } 
 public Group（）
 {
 RelatedNews = new List< News>（）; 
} 
  

在我的新闻控制器中，我添加：

  Group group = new Group（）; 
 group.RelatedNews.Add（news）; 
  

但没有更新，NewsID不会添加到我的GroupNews表。

解决方案

您不应该单独定义GroupNews。意思是，您不应该在项目中定义一个 GroupNews 类。您必须使用独立关联（新闻在组类中列出组在您的新闻类）。以下是您的课程应该如下所示：

  public class Group 
 {
 ... 
 public Group（）
 {
 this.News = new List< News>（）; 
} 
 
 public virtual ICollection< News>新闻{get; set;} 
} 
 
 public class新闻
 {
 ... 
 public News（）
 {
 this.Group = new List< Group>（）; 
} 
 public virtual ICollection< Group>组{get; set;} 
} 
 
 public class MyContext：DbContext 
 {
 public DbSet< Group>组{get;组; } 
 public DbSet< News>新闻{get;组; } 
} 
  

然后你可以使用 myGroup.News.Add （myNewsItem）或 myNews.Groups.Add（myGroup）。实体框架将自动处理插入。请注意，如果要为您的关联启用延迟加载，您应该使用 virtual 关键字。

I have a news entity and I get the news based on their NewsID. then I defined a new entity , a Group and I want to get the news based on their Group ID. I tried to handle this (many to many) relationships using this articleby using code first approach.

so in my context I added :

 public class Groupnews : DbContext
    {

        public DbSet<Group> Group { get; set; }
        public DbSet<News> News { get; set; }

        public Groupnews()
            : base("MyDb")
        {
        }

        public int NewsID { get; set; }
    }

this.HasMany(t => t.News)
    .WithMany(t => t.Groups)
    .Map(m =>
        {
            m.ToTable("GroupNews");
            m.MapLeftKey("GroupID");
            m.MapRightKey("NewsID");
        });

now I can get the news based on Their GroupID using this approach. but the problem is in insertign new News and updating.For that I need to save NewsID and GroupId in GroupNews table. for doing this . in News model i defined :

    public virtual ICollection<Group> RelatedGroups { get; set; }

    public News()
    {
        RelatedGroups = new List<Group>();
    }

and the same for group :

    public virtual ICollection<News> RelatedNews { get; set; }
    public Group()
    {
        RelatedNews = new List<News>();
     }

In my news controller I add :

            Group group = new Group();
            group.RelatedNews.Add(news);

but nothing is updated and the NewsID is not adding to my GroupNews table .

解决方案

You should not define GroupNews separately. Meaning, you should not have a GroupNews class defined in your project. You have to do CRUD operations using independent associations (a list of News in Group class and a list of Group in your News class). Here's what your classes should look like:

public class Group
{
    ...
    public Group()
    {
         this.News = new List<News>();
    }

    public virtual ICollection<News> News {get;set;}
}

public class News
{
    ...
    public News()
    {
         this.Group = new List<Group>();
    }
    public virtual ICollection<Group> Groups {get;set;}
}

public class MyContext : DbContext
{
    public DbSet<Group> Groups { get; set; }
    public DbSet<News> News { get; set; }
}

Then you can use myGroup.News.Add(myNewsItem) or myNews.Groups.Add(myGroup). Entity Framework will handle the insertion automatically. Notice you should use virtual keyword if you want to enable lazy loading for your associations.

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