c# - Entity Framework 一对多

Question

我正在使用 Entity Framework 并有以下类

class Student
{
 [Key]
 public virtual int StudentID {get; set;}
 public virtual string StudentName {get; set;}
 public virtual ICollection<Note> Notes {get; set;}
}
class Note
{
 [Key]
 public virtual int NoteID {get; set;}
 public virtual int StudentID {get; set;}
 public virtual string Message {get; set;}
}
class StudentDBContext:DbContext
{
 public DbSet<Student> Students { get; set; }
 public DbSet<Note> Notes { get; set; }
}

总而言之，我有一类学生，每个人都有很多笔记。现在，我想使用 Linq 检索并显示特定学生的所有笔记。所以我尝试一下

using (StudentDBContext a = new StudentDBContext())
{
 var b = from c in a.Student
         where c.StudentID == 1001
         select c;

 var currStudent = b.FirstOrDefault();
 Console.WriteLine(currStudent.StudentName);

  //display all the messages of the current student
  foreach (var currNote in currStudent.Notes)
  Console.WriteLine(currNote.Message);
}

在上面的代码中，我的 foreach block 总是失败，因为 Student.Notes 始终为 null。我是否缺少初始化 Student.Notes 并从数据库填充它的某些步骤？

Answer 1

您的类(class)学生和注释应该是公开的。

运行以下代码:

class Program {
    static void Main(string[] args) {
        using ( StudentDBContext efc = new StudentDBContext()) {
            foreach (var v in efc.Students) {
                Console.WriteLine("{0}", v.StudentName);
                foreach (var vv in v.Notes) {
                    Console.WriteLine("    {0}", vv.Message);
                }
            }
        }
    }
}

public class Student {
    public Student() {
        //Notes = new List<Note>();
    }
     [Key]
     public int StudentID {get; set;}
     public virtual string StudentName {get; set;}
     public virtual ICollection<Note> Notes {get; set;}
}
public class Note {
 [Key]
 public int NoteID {get; set;}
 public int StudentID {get; set;}
 public string Message {get; set;}
}
class StudentDBContext:DbContext {
    public DbSet<Student> Students { get; set; }
    public DbSet<Note> Notes { get; set; }        
}