c# - 系统.Data.SqlClient.SqlException : 'Introducing FOREIGN KEY constraint

Question

我使用 Entity Framework Core 和 Code First 方法，但在尝试启动应用程序时收到以下错误:

System.Data.SqlClient.SqlException: 'Introducing FOREIGN KEY constraint 'FK_Grades_Students_StudentId' on table 'Grades' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint or index. See previous errors.'

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

您在 Grade 上有一个不必要的外键。 Grade 引用 Subject 和 Subject 引用 Student。因此，当 Subject 也引用 Student 时，从 Grade 引用 Student 会创建冗余路径。从 Grade 或 Subject 中删除 Student。