异常:违反了多重性约束

Question

在将Question和PossibleAnswer的关系从Many-to-Many更改为One-to-Many后，我得到了一个异常："已成功提交对数据库的更改，但在更新对象上下文时出错。ObjectContext可能处于不一致状态。内部异常消息：违反了多重性约束。关系"WebUI.Models.PossibleAnswer_Question"的角色"PossibleAnswer_Question_Source"的多重性为1或0..1。"这是什么意思？

这是我的型号：

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; }
    }

并查看Question的模型（我知道ToQuestion应该在控制器中，稍后我会重新排列它）：

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;
        }
    }

这是我创建新问题的帖子方法：

[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);
}

线路：question.PossibleAnswers.Add(new PossibleAnswer { Text = possibleAnswer });导致异常，因为在将可能的答案添加到问题之前，我没有将其保存在数据库中。。。但是，我如何在不使用DbContext的情况下将它们添加到数据库中（因为在将视图模型转换为模型的方法中使用DbContext不是一个好的实践）？

Answer 1

由于PossibleAnswers未标记为已添加实体，因此将出现错误。

所以更新你的QuestionVM如下：

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;
    }
}

然后更新你的代码如下。

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