如何将新对象添加到现有实体中，PUT数据并让EF将其添加到我的数据库中

我使用的是EF5、SQL Server 2012和Web前端。在服务器上，我有以下两个类：

public class Question
{
    public Question()
    {
        this.Answers = new List<Answer>();
    }
    public int QuestionId { get; set; }
    public string Title { get; set; }
    public string Answer { get; set; }
    public virtual ICollection<Answer> Answers { get; set; }
}
public class Answer
{
    public int AnswerId { get; set; }
    public string Text { get; set; }
    public int QuestionId { get; set; }
    public virtual Question Question { get; set; }
}

我在前端应用程序中检索到一个问题，添加了一个空答案：

   var emptyAnswer = { "answerswerId"; null, "text": "xx", "questionId": formData.questionId};
   formData.answers.push(emptyAnswer);

然后我对服务器进行了PUT，并注意到当它到达服务器时，answerId被正确地设置为null。

    public HttpResponseMessage PutQuestion(int id, Question question)
    {
        var a = question;
    }

数据由我的ASP.NET WebApi控制器接收，但当我检查新创建的答案对象（现在是问题的一部分）时，它的AnswerId为0。

有人能告诉我为什么它从null变为0吗？问题是，当我现在更新实体时框架给了我一个错误，因为它期望新创建的答案没有设置它的AnswerId。

请注意。在对象到达实体框架之前，我正在进行检查。我提到我正在使用它，但在这一点上，我认为它还没有出现在画面中，因为我只是检查了poco类。下文提到该字段不可为null。然而，当我创建一个新对象时，有效负载是类似的，例如，当我新建一个"内容"对象时，我将字段设置为null，并且它保持为null。

这是我到目前为止的测试结果。我把前端从循环中去掉了，所以一切都更清楚了：

作品：

var a = new Answer{
    Correct = false,
    Response = false,
    Text = "AA",
    Image = "DDD",
    QuestionId = question.QuestionId
};
question.Answers.Add(a);
_uow.Questions.Update(question);
_uow.Commit();

不起作用：ObjectStateManager中已存在具有相同键的对象。ObjectStateManager无法跟踪具有相同键的多个对象。

var a = new Answer{
    AnswerId = 0,
    Correct = false,
    Response = false,
    Text = "AAA",
    Image = "DDD",
    QuestionId = question.QuestionId
};
var b = new Answer
{
    AnswerId = 0,
    Correct = false,
    Response = false,
    Text = "BBB",
    Image = "EEE",
    QuestionId = question.QuestionId
};

工作不正常。它不会创建新的实体值。它只使用1000和1001

var a = new Answer{
    AnswerId = 1000,
    Correct = false,
    Response = false,
    Text = "AAA",
    Image = "DDD",
    QuestionId = question.QuestionId
};
var b = new Answer
{
    AnswerId = 1001,
    Correct = false,
    Response = false,
    Text = "BBB",
    Image = "EEE",
    QuestionId = question.QuestionId
};

不起作用：编译器错误。无法将null转换为int

var a = new Answer{
    AnswerId = null,
    Correct = false,
    Response = false,
    Text = "AAA",
    Image = "DDD",
    QuestionId = question.QuestionId
};
var b = new Answer
{
    AnswerId = null,
    Correct = false,
    Response = false,
    Text = "BBB",
    Image = "EEE",
    QuestionId = question.QuestionId
};

不起作用：ObjectStateManager无法跟踪具有相同键的多个对象。

var a = new Answer{
    Correct = false,
    Response = false,
    Text = "AAA",
    Image = "DDD",
    QuestionId = question.QuestionId
};
var b = new Answer
{
    Correct = false,
    Response = false,
    Text = "BBB",
    Image = "EEE",
    QuestionId = question.QuestionId
};

public class Answer
{
    public int AnswerId { get; set; }
    public string Text { get; set; }
    public int QuestionId { get; set; }
    public virtual Question Question { get; set; }
}