INSERT语句与FOREIGN KEY约束“ FK_PostTag_Tag_TagId”冲突。 [英] The INSERT statement conflicted with the FOREIGN KEY constraint "FK_PostTag_Tag_TagId"

问题描述

我使用的是 Entity Framework 7 RC1 ，我有以下实体：

I am using Entity Framework 7 RC1 and I have the entities:

public class Post {
  public Int32 Id { get; set; }
  public String Title { get; set; }
  public virtual IList<PostTag> PostsTags { get; set; }
}

public class Tag {
  public Int32 Id { get; set; }
  public String Name { get; set; }
  public virtual IList<PostTag> PostsTags { get; set; }
}

public class PostTag {
  public Int32 PostId { get; set; }
  public Int32 TagId { get; set; }
  public virtual Post Post { get; set; }
  public virtual Tag Tag { get; set; }
}

这些实体的模型配置如下：

The model configuration for these entities is the following:

protected override void OnModelCreating(ModelBuilder builder) {

  base.OnModelCreating(builder);

  builder.Entity<Post>(b => {

    b.ToTable("Posts");
    b.HasKey(x => x.Id);
    b.Property(x => x.Id).UseSqlServerIdentityColumn();
    b.Property(x => x.Title).IsRequired().HasMaxLength(100);
  });

  builder.Entity<Tag>(b => {
    b.ToTable("Tags");
    b.HasKey(x => x.Id);
    b.Property(x => x.Id).UseSqlServerIdentityColumn();
    b.Property(x => x.Name).IsRequired().HasMaxLength(100);
  });

  builder.Entity<PostTag>(b => {
    b.ToTable("PostsTags");
    b.HasKey(x => new { x.PostId, x.TagId });
    b.HasOne(x => x.Post).WithMany(x => x.PostsTags).HasForeignKey(x => x.PostId);
    b.HasOne(x => x.Tag).WithMany(x => x.PostsTags).HasForeignKey(x => x.TagId);
  });

}

我创建了迁移和数据库。然后我尝试创建一个帖子：

I created the migration and the database. Then I tried to create a post:

  Context context = new Context();

  Post post = new Post {
    PostsTags = new List<PostTag> {
      new PostTag {
        Tag = new Tag { Name = "Tag name" }
      }
    },
    Title = "Post title"
  };

  context.Posts.Add(post);

  await _context.SaveChangesAsync();

在保存时，我得到以下错误：

And on save I get the following error:

An error occurred while updating the entries. 
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_PostTag_Tag_TagId". 
The conflict occurred in database "TestDb", table "dbo.Tags", column 'Id'. 
The statement has been terminated.

有人知道这个错误的原因吗？

Does anyone knows the reason for this error?

推荐答案

我遇到了同样的问题。这是我想出的解决方案。 这个 SO问题对我有很大帮助。

I had the same problems. Here's the solution I came up with. This SO question helped me a lot.

首先，添加 public DbSet< Tag>标签{get;设置；} 到您的 Context 类（如果缺少）。

First of all, add a public DbSet<Tag> Tags {get; set;} to yout Context class if it's missing.

然后修改帖子创建如下

Context context = new Context();
var tmpTag = new Tag { Name = "Tag name" } //add the tag to the context
context.Tags.Add(tmpTag);

Post post = new Post {
    PostsTags = new List<PostTag>(), // initialize the PostTag list
    Title = "Post title"
};    
context.Posts.Add(post);

var postTag = new PostTag() {Post = post, Tag = tag}; // explicitly initialize the PostTag AFTER addig both Post and Tag to context
post.PostTags.Add(postTag); // add PostTag to Post

await _context.SaveChangesAsync();

同时添加 post 和标签到 context.Posts 和 context.Tags 之前， PostTag 对象允许EF在写入基础数据库时正确管理ID。

Explictly adding both post and tag to context.Posts and context.Tags before attempting to create the PostTag object allows EF to correctly manage the IDs while writing to the underlying DB.

为了完整性，解决了多对多关系管理的这一部分之后，我目前正在使用CascadeDelete Entity Framework Core（EF7）挣扎，但这是另一回事。

For the sake of completeness, after solving this part of the many-to-many relationship management, I'm currently struggling with CascadeDelete Entity Framework Core (EF7), but that's a different story.

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