实体框架 - 3 个表之间有关系

Question

我有3张表如下：

应用程序用户：

public class ApplicationUser : IdentityUser
{
    ..some basic properties..

    // navigation properties
    public virtual ICollection<Post> Posts { get; set; }
    public virtual ICollection<Album> Albums { get; set; }
}

发帖：

public class Post
{
        public long Id { get; set; }    
        public string Content { get; set; }

        public int? AlbumId { get; set; }
        public string UserId { get; set; }

        public virtual ApplicationUser User { get; set; }
        public virtual Album Album { get; set; }
}

专辑：

public class Album
{
        public int Id { get; set; }
        public string Name { get; set; }
        public string UserId { get; set; }

        public virtual ApplicationUser User { get; set; }
        public virtual ICollection<Post> Posts { get; set; }
}

最后是 ApplicationDbContext：

modelBuilder.Entity<ApplicationUser>()
                .HasMany(a=>a.Posts)
                .WithRequired(a=>a.User)
                .HasForeignKey(a=>a.UserId)
                .WillCascadeOnDelete(false);
modelBuilder.Entity<Post>()
                .HasKey(p => p.Id);

modelBuilder.Entity<Album>()
                .HasKey(a => a.Id);

modelBuilder.Entity<ApplicationUser>()
                .HasMany(u=>u.Albums)
                .WithOptional()
                .HasForeignKey(a=>a.UserId)
                .WillCascadeOnDelete();

modelBuilder.Entity<Album>()
                .HasMany(a=>a.Posts)
                .WithRequired()
                .HasForeignKey(p=>p.AlbumId)
                .WillCascadeOnDelete();

当我运行迁移和更新数据库时，我得到一个错误：

ALTER TABLE 语句与 FOREIGN KEY 约束冲突 “FK_dbo.Posts_dbo.Albums_AlbumId”。数据库发生冲突 “aspnet-Link-20161012104217”，表“dbo.Albums”，列“Id”。

谁能告诉我他们为什么会发生冲突？这对我来说似乎很合法。

Answer 1

在您的代码中，您将AlbumId 设置为nullable，但在配置中定义了WithRequeired()：

public class Post
{
    public long Id { get; set; }    
    public string Content { get; set; }

    public int? AlbumId { get; set; }       //<-- this one
    public string UserId { get; set; }

    public virtual ApplicationUser User { get; set; }
    public virtual Album Album { get; set; }
}

modelBuilder.Entity<Album>()
            .HasMany(a=>a.Posts)
            .WithRequired() //<-- this one
            .HasForeignKey(p=>p.AlbumId)
            .WillCascadeOnDelete();

如果AlbumId 是nullable 你应该改变配置：

//Ef by default conventions set the AlbumId as foreign key
modelBuilder.Entity<Album>()
            .HasMany(a=>a.Posts)
            .WithOptional(a=>a.Album);

如果AlbumId 不是nullable，则更改属性：

public class Post
{
    public long Id { get; set; }    
    public string Content { get; set; }

    public int AlbumId { get; set; }     
    public string UserId { get; set; }

    public virtual ApplicationUser User { get; set; }
    public virtual Album Album { get; set; }
}

并使用以下配置：

//Ef by default conventions set the AlbumId as foreign key
modelBuilder.Entity<Album>()
            .HasMany(a=>a.Posts)
            .WithRequired(a=>a.Album)
            .WillCascadeOnDelete();