重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/Contracts/Class.cs

@@ -10,19 +10,24 @@ namespace Entities.Contracts
 	{
 		[Key]
 		[Column("class_id")]
-		public int ClassId { get; set; }
+		public Guid Id { get; set; }
 
 		[Column("grade_id")]
-		public int GradeId { get; set; }
-		public Grade Grade { get; set; }
+		public Guid GradeId { get; set; }
+		[ForeignKey(nameof(GradeId))]
+		public virtual Grade Grade { get; set; }
+
+		[Column("index")]
+		public byte Index { get; set; }
 
 		[Column("class_name")]
 		[MaxLength(30)]
 		public string ClassName { get; set; }
 
 		[Column("head_teacher_id")]
-		public int? HeadTeacherId { get; set; }
-		public User HeadTeacher { get; set; }
+		public Guid? HeadTeacherId { get; set; }
+		[ForeignKey(nameof(HeadTeacherId))]
+		public virtual User HeadTeacher { get; set; }
 
 		[Column("created_at")]
 		public DateTime CreatedAt { get; set; }
@@ -34,15 +39,20 @@ namespace Entities.Contracts
 		public bool IsDeleted { get; set; }
 
 		// Navigation Properties
-		public ICollection<ClassTeacher> ClassTeachers { get; set; }
-		public ICollection<ClassStudent> ClassStudents { get; set; }
-		public ICollection<AssignmentClass> AssignmentClasses { get; set; }
+
+		[NotMapped]
+		public ICollection<ClassUser> Techers => ClassUsers.Where(cu => cu.User.Role == UserRoles.Teacher).ToList();
+
+		[NotMapped]
+		public ICollection<ClassUser> Students => ClassUsers.Where(cu => cu.User.Role == UserRoles.Student).ToList();
+
+		[InverseProperty(nameof(ClassUser.Class))]
+		public virtual ICollection<ClassUser> ClassUsers { get; set; }
 
 		public Class()
 		{
-			ClassStudents = new HashSet<ClassStudent>();
-			ClassTeachers = new HashSet<ClassTeacher>();
-			AssignmentClasses = new HashSet<AssignmentClass>();
+			Id = Guid.NewGuid();
+			ClassUsers = new HashSet<ClassUser>();
 			CreatedAt = DateTime.Now;
 			UpdatedAt = DateTime.Now;
 		}