系统数据SqlClient。SqlException：“引入外键约束[重复]

Question

我正在使用实体框架核心和代码优先的方法，但在尝试启动应用程序时收到以下错误：

系统数据SqlClient。SqlException：“在表“Grades”上引入外键约束“FK\U Grades\U StudentId”可能会导致循环或多个级联路径。指定ON DELETE NO ACTION或ON UPDATE NO ACTION，或修改其他外键约束。无法创建约束或索引。请参阅以前的错误。”

我的实体：

public class Student
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string Password { get; set; }

    public string Email { get; set; }

    public Group Group { get; set; }

    public IEnumerable<Subject> Subjects { get; set; } = new List<Subject>();


}

public class Subject
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public string Name { get; set; }

    public IEnumerable<Grade> Grades { get; set; } = new List<Grade>();

    [ForeignKey("StudentId")]

    public Student Student { get; set; }

    public Guid StudentId { get; set; }

}

public class Grade
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public double Value { get; set; }

    public DateTime DateOfGettingGrade { get; set; }

    [ForeignKey("SubjectId")]

    public Subject Subject { get; set; }

    public Guid SubjectId { get; set; }

    [ForeignKey("StudentId")]

    public Student Student { get; set; }

    public Guid StudentId { get; set; }
}


modelBuilder.Entity("GradeBook.Core.Models.Grade", b =>
            {
                b.HasOne("GradeBook.Core.Models.Student", "Student")
                    .WithMany()
                    .HasForeignKey("StudentId")
                    .OnDelete(DeleteBehavior.Cascade);

                b.HasOne("GradeBook.Core.Models.Subject", "Subject")
                    .WithMany("Grades")
                    .HasForeignKey("SubjectId")
                    .OnDelete(DeleteBehavior.Cascade);
            });

Answer 1

多个级联路径的原因是，您可以通过两种方式从Grade到Subject，或者Grade-

Answer 2

你有一个不必要的外键在等级。等级引用主题和主题引用学生。所以当主题也引用学生时，从等级引用学生会创建一个冗余路径。从等级或主题中删除学生。