[英]Entity Framework: select records that don't exist in relationship
我正在努力獲取缺少必修課程的學生名單。
我有一個使用連接表的多對多關系:
學生:
public class Student: IBaseEntity
{
[Key]
public Guid ID { get; set; }
private string _name = string.Empty;
public string Name
{
get => _name;
set => _name = value!.ToUpper(CultureInfo.InvariantCulture);
}
// removed additional fields for brevity
}
課程:
public class Course: IBaseEntity
{
[Key]
public Guid ID { get; set; }
private string _name = string.Empty;
public string Name
{
get => _name;
set => _name = value!.ToUpper(CultureInfo.InvariantCulture);
}
public bool Required { get; set; }
// removed additional fields for brevity
}
行動:
public class Action: IBaseEntity
{
[Key]
public Guid ID { get; set; }
public Student Student { get; set; }
public Course Course { get; set; }
// the date that the student took the course
public Datetime AttendedDate { get; set; }
// removed additional fields for brevity
}
如何獲取缺少必修課程的學生列表?
例如作為Dictionary<string, Student>
,其中 string 是課程和沒有參加課程的學生。
我嘗試了一個foreach(var s in Students)
和一個foreach(var c in courses)
,但它掛起......
Dictionary<string, Student> dict = new();
foreach (var s in Students)
{
foreach (var c in Courses)
{
bool exists = actions.Any(x => x.Student.ID.Equals(c.ID) && x.Course.ID.Equals(c.ID));
if (!exists)
{
dict.Add(r.Name, c);
}
}
}
也許我以錯誤的方式處理關系?
任何幫助表示贊賞。
PS:對不起我的英語,不是我的母語。
您必須測試x.Student.ID.Equals(s.ID)
而不是c.ID
。
而且您不能將重復的鍵添加到字典中。 例如,將s
和缺少的c
添加到List<(Student student, Course course)>
中,其中列表包含元組。
List<(Student student, Course course)> missing = new();
foreach (var s in Students) {
foreach (var c in Courses) {
bool exists = actions.Any(a => a.Student.ID == s.ID && a.Course.ID == c.ID);
if (!exists) {
missing.Add((s, c));
}
}
}
foreach (var (student, course) in missing) {
Console.WriteLine($"{student.Name} did not attend \"{course.Name}\"");
}
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