Exception: Multiplicity constraint violated

Question

After changing relationship of Question and PossibleAnswer from Many-to-many to One-to-many I got an exception: "The changes to the database were committed successfully, but an error occurred while updating the object context. The ObjectContext might be in an inconsistent state. Inner exception message: Multiplicity constraint violated. The role 'PossibleAnswer_Question_Source' of the relationship 'WebUI.Models.PossibleAnswer_Question' has multiplicity 1 or 0..1." What does it mean?

Here is my model:

public class Question
    {
        public int ID { get; set; }
        public string Text { get; set; }
        public bool IsAssociatedWithProfessor { get; set; }
        public bool IsAssociatedWithAssistant { get; set; }

        public virtual ICollection<PossibleAnswer> PossibleAnswers { get; set; }
        public virtual ICollection<Results> Results { get; set; }
    }
public class PossibleAnswer
    {
        public int ID { get; set; }
        public string Text { get; set; }

        public virtual Question Question { get; set; }
    }

And view model for Question (I know that ToQuestion should be in controller, I will rearrange it later):

public class QuestionVM
    {
        public QuestionVM() {
        }
        public QuestionVM(Question question) : this()
        {
            ID = question.ID;
            Text = question.Text;
            IsAssociatedWithProfessor = question.IsAssociatedWithProfessor;
            IsAssociatedWithAssistant = question.IsAssociatedWithAssistant;
        }
        public int? ID { get; set; }
        public string Text { get; set; }
        public bool IsAssociatedWithProfessor { get; set; }
        public bool IsAssociatedWithAssistant { get; set; }
        private IEnumerable<string> _possibleAnswers;
        public IEnumerable<string> PossibleAnswers 
        { 
            get
            {
                return _possibleAnswers ?? new List<string>(){"", "", "", "", ""};
            }
            set 
            {
                _possibleAnswers = value;
            }
        }
        public Question ToQuestion()
        {
            Question question = new Question
            {
                Text = this.Text,
                IsAssociatedWithProfessor = this.IsAssociatedWithProfessor,
                IsAssociatedWithAssistant = this.IsAssociatedWithAssistant,
                PossibleAnswers = new List<PossibleAnswer>()
            };
            //ID will be null if creating new question
            if(ID != null)
            {
                question.ID = (int) ID;
            }
            foreach (string possibleAnswer in this.PossibleAnswers)
            {
                if (!String.IsNullOrEmpty(possibleAnswer))
                {
                    question.PossibleAnswers.Add(new PossibleAnswer { Text = possibleAnswer });
                }
            }
            return question;
        }
    }

Here is my post method for creating new question:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult AddQuestion(QuestionVM questionVM)
{
    try
    {
        if (ModelState.IsValid)
        {
            Question question = questionVM.ToQuestion();
            context.Questions.Add(question);
            context.SaveChanges();
            return RedirectToAction("Questions");
         }
     }
     catch (DataException /* dex */)
     {
        //Log the error (uncomment dex variable name and add a line here to write a log.
        ModelState.AddModelError("", "Trenutno nije moguće snimiti promjene, pokušajte ponovo.");
     }
     return View(questionVM);
}

Line:question.PossibleAnswers.Add(new PossibleAnswer { Text = possibleAnswer }); causes the exception since I didn't save possible answers in database before I am adding them to question... But how can I add them to database without use of DbContext (since it is not a good practice to use DbContext in method which converts view model to model)?

Answer 1

Error is coming because PossibleAnswers are not being marked as added entity.

So update your QuestionVM like below:

public class QuestionVM
{
    public QuestionVM() {
    }
    public QuestionVM(Question question) : this()
    {
        ID = question.ID;
        Text = question.Text;
        IsAssociatedWithProfessor = question.IsAssociatedWithProfessor;
        IsAssociatedWithAssistant = question.IsAssociatedWithAssistant;
    }
    public int? ID { get; set; }
    public string Text { get; set; }
    public bool IsAssociatedWithProfessor { get; set; }
    public bool IsAssociatedWithAssistant { get; set; }
    private IEnumerable<string> _possibleAnswers;
    public IEnumerable<string> PossibleAnswers 
    { 
        get
        {
            return _possibleAnswers ?? new List<string>(){"", "", "", "", ""};
        }
        set 
        {
            _possibleAnswers = value;
        }
    }
    public Question ToQuestion()
    {
        Question question = new Question
        {
            Text = this.Text,
            IsAssociatedWithProfessor = this.IsAssociatedWithProfessor,
            IsAssociatedWithAssistant = this.IsAssociatedWithAssistant,
            PossibleAnswers = new List<PossibleAnswer>()
        };
        //ID will be null if creating new question
        if(ID != null)
        {
            question.ID = (int) ID;
        }
        return question;
    }

    public List<PossibleAnswer> GetPosibleAnswers()
    {
        var listOfPossibleAnswers = new List<PossibleAnswer>();
        foreach (string possibleAnswer in this.PossibleAnswers)
        {
            if (!String.IsNullOrEmpty(possibleAnswer))
            {
                listOfPossibleAnswers.Add(new PossibleAnswer { Text = possibleAnswer });
            }
        }
        return listOfPossibleAnswers;
    }
}

and then update your code like below.

    if (ModelState.IsValid)
    {
        Question question = questionVM.ToQuestion();
        context.Questions.Add(question);
        context.SaveChanges();
        question.PossibleAnswers.AddRange(questionVM.GetPosibleAnswers());
        context.SaveChanges();
        return RedirectToAction("Questions");
     }