Entity Framework Code first - FOREIGN KEY 约束问题

Question

我是 EF 代码第一主体的新手，目前不知道该怎么做。我有 2 个 POCO 课程。

public class Problem
{
    public int ProblemID { get; set; }
    public int UserID { get; set; }
    public int CategoryID { get; set; }
    public int RatingID { get; set; }

    public string Title { get; set; }
    public string Description { get; set; }        
    public double Latitude { get; set; }
    public double Longitude { get; set; }
    public int State { get; set; }
    public DateTime CreatedOn { get; set; }

    public virtual Rating Rating { get; set; }
    public virtual Category Category { get; set; }
    public virtual User User { get; set; }

    public virtual ICollection<Picture> Pictures { get; set; }
    public virtual ICollection<Comment> Comments { get; set; }
}

和

public class User
{
    public int UserID { get; set; }
    public int RoleID { get; set; }

    public string Name { get; set; }
    public string Surname { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string SocialHandle { get; set; }       
    public DateTime RegisteredOn { get; set; }

    public virtual Role Role { get; set; }

    public virtual ICollection<Point> Points { get; set; } 
    public virtual ICollection<Problem> Problems { get; set; }
    public virtual ICollection<Comment> Comments { get; set; }               
}

我的数据库创建正常，但是当我尝试使用自定义初始化程序在其中初始化一些数据时，我收到以下错误：

在表上引入 FOREIGN KEY 约束“Problem_User” “问题”可能导致循环或多个级联路径。指定开 DELETE NO ACTION 或 ON UPDATE NO ACTION，或修改其他 FOREIGN KEY 约束。
无法创建约束。查看以前的错误。

这是我的初始化程序：

protected override void Seed(CleanStreets context)
    {
        var adminRole = new Role { Name = "Administrator", RoleID = 0 };            
        var moderatorRole = new Role { Name = "Moderator", RoleID = 1 };
        var userRole = new Role { Name = "User", RoleID = 2 };

        context.Roles.Add(adminRole);
        context.Roles.Add(userRole);
        context.Roles.Add(moderatorRole);

        var admin = new User { Name = "admin", Surname = "admin", Role = adminRole, Email = "fake@fake.com", Password = "", RegisteredOn = DateTime.Now, SocialHandle = null };
        var user = new User { Name = "user", Surname = "user", Role = userRole, Email = "fake@fake2.com", Password = "", RegisteredOn = DateTime.Now, SocialHandle = null };

        context.Users.Add(admin);
        context.Users.Add(user);

        var categoryOne = new Category { Title = "Komunalni problemi", CategoryID = 0 };
        var categoryTwo = new Category { Title = "Ostali problemi", CategoryID = 1 };

        context.Categories.Add(categoryOne);
        context.Categories.Add(categoryTwo);

        var problem = new Problem { Category = categoryOne, Title = "Prvi testni problem", Description = "Ovo je testni opis", Latitude = 45.5, Longitude = 15.5, State = 0, CreatedOn = DateTime.Now, User = user };

        context.Problems.Add(problem);

        base.Seed(context);
    }

我做错了什么？提前谢谢！

Answer 1

那是因为Comments。默认情况下，EF 对引用使用级联删除。在您的情况下，级联删除将从用户 -> 问题，用户 -> 评论创建，但也从问题 -> 评论创建。如果您删除了级联到同一评论记录的用户，则可能来自Problem 和User。这在 SQL Server 中是不允许的。每条记录只能通过单个级联删除路径访问。

为避免这种情况，您必须使用流畅的映射来对一个关系进行级联删除（您必须选择哪一个）。用户示例 -> 评论

public class Context : DbContext
{
    public DbSet<User> Users { get; set; }
    public DbSet<Comment> Comments { get; set; }
    public DbSet<Problem> Problems { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
                    .HasMany(u => u.Comments)
                    .HasRequired(c => c.User)
                    .HasForeignKey(c => c.UserId)
                    .WillCascadeOnDelete(false);

        base.OnModelCreating(modelBuilder);
    }
} 

在您的模型中可能存在可能导致此问题的其他实体和关系，但 Comment 从您的示例中看起来很明显。