问更改asp.net mvc模型属性时无法访问数据库

EN

public class SchoolInfoEntities: DbContext
    {
        public DbSet<Students> Student { get; set; }
        public DbSet<Class> Classes { get; set; }
        public DbSet<Guardian> Guardians { get; set; }
        public DbSet<Staff> Staffs { get; set; }
        public DbSet<Subject> Subjects { get; set; }
        public DbSet<Department> Departments { get; set; }
        public DbSet<SchoolDetails> SchoolDetails { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            
            base.OnModelCreating(modelBuilder);

            modelBuilder.Entity<Students>()
                .HasMany(t => t.Guardians)
                .WithMany(t => t.Students)
                .Map(m =>
               {
                   m.ToTable("Admission.StudentGuardian");
                   m.MapLeftKey("StudentId");
                   m.MapRightKey("GuardianId");
               });

            modelBuilder.Entity<Staff>()
                .HasMany(t => t.Subjects)
                .WithMany(t => t.Staffs)
                .Map(m =>
                {
                    m.ToTable("Admission.SubjectInstructor");
                    m.MapLeftKey("StaffId");
                    m.MapRightKey("SubjectName");
                });

            modelBuilder.Entity<Staff>()
                .HasMany(t => t.Departments)
                .WithMany(t => t.Staffs)
                .Map(m =>
                {
                    m.ToTable("Admission.StaffDepartment");
                    m.MapLeftKey("StaffId");
                    m.MapRightKey("DepartmentId");
                });

            Database.SetInitializer<SchoolInfoEntities>(null);

        }
    }