使用C＃代码中的LINQ读取icollection数据

Question

我需要使用Linq查询获取Icollection数据以及每个记录实例的其他数据，我正在做的事情。

课程模型

public partial class Course
{
    public Course()
    {
        this.StudentCourses = new HashSet<StudentCourse>();
    }

    public int CourseID { get; set; }
    public string Title { get; set; }

    public virtual ICollection<StudentCourse> StudentCourses { get; set; }
}

学生模特

   public partial class Student
{
    public Student()
    {
        this.StudentCourses = new HashSet<StudentCourse>();
    }

    public int StudentID { get; set; }
    public string Name { get; set; }

    public virtual ICollection<StudentCourse> StudentCourses { get; set; }
}

学生课程模型

public partial class StudentCourse
{
    public int StudentCourseID { get; set; }
    public int StudentID { get; set; }
    public int CourseID { get; set; }

    public virtual Course Course { get; set; }
    public virtual Student Student { get; set; }
}

方法读取记录

   public void readStudents()
    {
        using (var db2 = new MyDbContext())
        {
            List<Student> _studentRead = new List<Student>();

            _studentRead = (from b in db2.Students
                            orderby b.StudentID
                            select b).ToList() ;

        }
    }

班级模特

 public partial class Student
{
    public Student()
    {
        this.StudentCourses = new HashSet<StudentCourse>();
    }

    public int StudentID { get; set; }
    public string Name { get; set; }

    public virtual ICollection<StudentCourse> StudentCourses { get; set; }
}

问题的第3部分

我现在得到的结果是正确的;我可以使用以下声明看到学生和他们注册的课程列表;

  using (var db2 = new MyDbContext())
        {
            List<Student> _studentRead = new List<Student>();

            _studentRead = (from _student in db2.Students.Include(r=>r.StudentCourses)        
                            select _student).ToList();
        }

现在我如何在我的linq语句中包含课程标题如果我想如下图所示我可以阅读课程

Answer 1

如果您在加载学生对象时尝试加载StudentCourse集合，则可以使用Include扩展方法。

public void LoadStudents()
{
  using (var db2 = new MyDbContext()
  {
    //Lambda Linq
    var studentList = new List<Student>();
    studentList = db2.Students
                     .Include(s => s.StudentCourses)
                     .OrderBy (s => s.StudentId)
                     .ToList();

    //Comprehension Linq 
    var compList = new List<Student>();
    compList = (from s in db2.Students.Include(r => r.StudentCourses)
               OrderBy s.StudentId
               Select s).ToList();
  }
}

这将明确告诉上下文在从上下文加载每个学生对象时加载集合。 扩展方法位于System.Data.Entity命名空间

中

可在此处找到更多信息： https://msdn.microsoft.com/en-us/data/jj574232.aspx

Upate 1 - 显示加载多个依赖对象级别

public void LoadStudents()
{
  //Lambda Linq
  var lambdaList = db2.Students
                      .Include(s => s.StudentCourses.Select(sc => sc.Course))
                      .OrderBy(s => s.StudentId)
                      .ToList();

 //Comprehension Linq
 var compList = (from student in db2.Students.Include(s => s.StudentCourse.Select(sc => sc.Course)
                 OrderBy student.StudentId
                 Select student).ToList(); 
}