CODEWITHSUNDEEP
c#linqentity-framework-4many-to-many
Only Bolivian Here
Only Bolivian Here

Reputation: 36743

How can I retrieve a List of entities from a many to many relationship?

I'm trying to retrieve a List<Student> from my database.

Basically, the schema is:

GradeParalelo ---- GradeStudent ---- Student

Where the GradeStudent table holds primary keys for GradeParalelo and Student. NOTE: The GradeStudent table has an extra column to hold an integer. This is not just an association table.

Here's what I've tried so far:

ColegioDBV2Entities db = new ColegioDBV2Entities();
public List<Student> FindAllStudentsFromGradeParalelo(int gradeParaleloId)
{
    return (db.Students
           .Include("GradeStudents")
           .Include("GradeStudents.GradeParalelo")
           .Where<Student>(s => s.StudentId == gradeParaleloId)).ToList();
}

But it's retrieving a single student for every gradeParalelo I choose. Understandably, because I'm comparing a StudentId with a GradeParaleloId. This is not what I want.

Can anyone suggest a way to retrieve this collection?

Image that I want to retrieve all students that are in the GradeStudent table with GradeParalelo ID 6.

Basically this, in a more elegant form:

public List<Student> FindAllStudentsFromGradeParalelo(int gradeParaleloId)
{
    List<Student> students = new List<Student>();

    using(GradeStudentRepository repo = new GradeStudentRepository())
    {
        var items = repo.FindAllGradeStudents().Where(g => g.GradeParaleloId == gradeParaleloId);

        StudentRepository studentRepo = new StudentRepository();
        foreach (var o in items)
        {
            students.Add(studentRepo.FindStudent(o.StudentId));
        }
    }

    return students;
}

Upvotes: 1

Views: 124

Answers (2)

SLaks
SLaks

Reputation: 887453

How about

return db.GradeParaleloes.Single(g => g.GradeParaleloId == gradeParaleloId)
                         .GradeStudents.Select(s => s.Student).ToList();

Upvotes: 3

balexandre
balexandre

Reputation: 75083

that is a simple query as:

return
    db.Students
        .Where( x => x.GradeParalelo.gradeParaleloId == gradeParaleloId )
        .ToList();

if you have the FK right, you just need to call the FK and internally it will link all the needed tables.

Upvotes: 1

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