CODEWITHSUNDEEP
c#asp.net-mvcentity-framework
Matthijs
Matthijs

Reputation: 3170

Objects don't seem to get saved into database

I am using Entity Framework code-first and the following two models:

public class Exam
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime CreationDate { get; set; }
    public ICollection<Question> Questions { get; set; }
}

public class Question
{
    public enum Answers
    {
        A,
        B,
        C,
        D
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public string AnswerA { get; set; }
    public string AnswerB { get; set; }
    public string AnswerC { get; set; }
    public string AnswerD { get; set; }
    public Answers Correct { get; set; }
}

The two models above show a relationship between Exam and Question; Exam has multiple Questions. To my understanding this relationship gets correctly created by Entity Frameworks' code-first.

However, when I try to read the questions inside an Exam object (after supposedly being added to the database), the Exam object shows Null for the Questions collection. Why does this happen? The object gets correctly created using the following create method. I have debugged through its process and the Exam object gets its questions assigned correctly.

public ActionResult Create(ExamVM examVM)
{
    // Return if no questions were selected.
    if (examVM.Questions == null)
        return View(examVM);

    // Create database-exam object.
    QuestionaireDbContext db = new QuestionaireDbContext();
    Exam exam = new Exam();
    exam.Questions = new List<Question>();

    // Fill exam with selected questions.
    foreach (QuestionVM questionVM in examVM.Questions.Where(q => q.IsSelected))
        exam.Questions.Add(db.Questions.Find(questionVM.ID));

    exam.Name = examVM.Name;
    exam.CreationDate = DateTime.Today;

    // Validate exam.
    if (ModelState.IsValid)
    {
        db.Exams.Add(exam);
        db.SaveChanges();
        // Return to index if changes were applied.
        return RedirectToAction("Index");
    }

    // Return if exam isn't valid.
    return View(examVM);
}

Note that in the above method I use a "helper" model (ExamVM) to assign the questions to the exam.

To repeat the question: why does my Exam object return Null for its Questions collection when querying from the database, while it gets correctly created and supposedly also correctly added to the database?

I read the exam using the following method:

public ActionResult TakeExam(int id)
{
     QuestionaireDbContext db = new QuestionaireDbContext();
     Exam exam = db.Exams.Find(id);
     return View("TakeExam", exam);
}

The exam holds the correct Name etc, but the collection shows to be null.

This is the DbContext, for reference:

public class QuestionaireDbContext : DbContext
{
    public DbSet<Question> Questions { get; set; }
    public DbSet<Exam> Exams { get; set; }
}

Upvotes: 1

Views: 67

Answers (1)

CodeCaster
CodeCaster

Reputation: 151594

You either need to enable lazy loading by making the Questions property virtual, or explicitly load the collection using 

dbContext.Exams.Include(e => e.Questions)

Upvotes: 2

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