INSERT 语句与 FOREIGN KEY 约束“FK_PostTag_Tag_TagId”冲突

我在用实体框架 7 RC1我有实体：

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：

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：

  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();

保存时出现以下错误：

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.

有谁知道这个错误的原因？

我想说的是，您不需要在 EF CodeFirst 中显式声明外键，框架将为您处理它。因此从 PostTag 类中删除这些属性

public Int32 PostId { get; set; }
public Int32 TagId { get; set; }

然后从配置中删除这两行，然后再次尝试保存。您可能需要在保存之前更新数据库模型。

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);