使用实体框架时如何填充导航属性

Question

I am using Entity Framework, Database first and need a good solution to the below problem I have an Entity say Courses generated by Entity Framework

class Courses
{
     public Courses()
     {
         this.Students=new HashSet<Student>();
     }
     public int courseid{get;set;}
     public virtual ICollection<Student> Students{get;set}
}

Class Student
{
     public int id{get;set;}
     public string Name{get;set;}
}

i created Model classes in my business layer which corresponds to these Entity Classes

 class Courses_Model
 {
     public Courses()
     {
        this.Students=new HashSet<Student>();
     }
     public int courseid{get;set;}
     public virtual ICollection<Student> Students{get;set}
}

Class Student_Model
{
     public int id{get;set;}
     public string Name{get;set;}
}

i want to return the model class(Courses_Model) from my web api method which should include navigation property students instead of

public Courses GetCourses(string id)
{ 
    var course= (from g in con.Courses.Include("Student") where g.REQUESTER_REFERENCE == id select g).First();

    return course;
}

To return Courses_Model we can create new Courses_Model object while returning like, but i am not sure how to populate

public Courses_Model GetCourses(string id)
{ 
    Course= con.Courses.Include("Student").Select(g => new Courses_Model{courseid=g.courseid }).Where(g => g.REQUESTER_REFERENCE == id).First();

    return course;
}

Answer 1

In your projection you need to instantiate Student_Model as well (also I would suggest projecting after Where and using lambda expressions rather than strings for Include ):

public Courses_Model GetCourses(string id)
{ 
    Course= con.Courses.Include(x => x.Students)
            .Where(g => g.REQUESTER_REFERENCE == id)
            .Select(
                  g => new Courses_Model
                           {
                                courseid = g.courseid,
                                Students = g.Students.Select(x => new Student_Model { id = x.id, Name = x.Name })
                           })
            .First();

    return course;
}

On a side note, libraries like AutoMapper are great for abstracting this kind of thing.

Answer 2

below is a simple sample I created for you

public class STUDENTS
{
    public STUDENTS()
    {
        COURSES = new List<COURSES>();
    }
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ST_ROWID { get; set; }

    public int ST_NAME { get; set; }

    [ForeignKey("CR_SM_REFNO")]
    public virtual List<COURSES> COURSES { get; set; }
}

public class COURSES
{
    [Key,DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int CR_ROWID { get; set; }

    public string CR_NAME { get; set; }

    public int CR_SM_REFNO { get; set; }

    [ForeignKey("CR_SM_REFNO")]
    public virtual STUDENTS STUDENTS { get; set; }
}

and the following methods do the job:

// gets the list of courses taken by the student id 
public List<COURSES> GetCoursesByStudent(int pST_ROWID)
    {
        using (var con = new MPContext())
        {
            return con.COURSES.Include(x=>x.STUDENTS).
                                Where(x => x.CR_SM_REFNO.Equals(pST_ROWID)).ToList();
        }
    }

    //Gets the list of students who get the course with the course id
    public List<STUDENTS> GetStudentsByCourse(int pCR_ROWID)
    {
        using (var con = new MPContext())
        {
            return con.STUDENTS.Include(x => x.COURSES).
                Where(x => x.COURSES.Any(y=>y.CR_ROWID.Equals(pCR_ROWID))).ToList();
        }
    }