Entity Framework 删除具有外键关系的实体时出错

Question

由于外键关系，我在删除某些实体时遇到问题。我了解以下错误消息，并已尽我所能删除实体而不会引发此错误：

The DELETE statement conflicted with the REFERENCE constraint "FK_QuizUserAnswer_QuizWithQuestion". The conflict occurred in database "SomeDatabase", table "dbo.QuizUserAnswer", column 'idQuizQuestion'. The statement has been terminated.

这是有问题的两个表的图片：

我正在尝试删除 QuizWithQuestion 个实体。我已将 idQuizQuestion 列设置为 nullable。因此，外键在 QuizUserAnswer 端可以为空。在映射文件中，我指定关系是 optional:

HasMany(t => t.QuizUserAnswers)
    .WithOptional(t => t.QuizWithQuestion)
    .HasForeignKey(t => t.idQuizQuestion);

HasOptional(t => t.QuizWithQuestion)
    .WithMany(t => t.QuizUserAnswers)
    .HasForeignKey(d => d.idQuizQuestion);

我已经尝试了很多很多代码片段，所以我将 post 代码的当前状态，希望我的意图是明确的：

    public void RemoveQuestionsFromQuiz(IEnumerable<int> deletedQuestions, int quizId)
    {
        var quiz = // code which retrieves quiz

        foreach (var deletedQuestion in deletedQuestions)
        {
            var quizWithQuestion = quiz.QuizWithQuestions.FirstOrDefault(q => q.Id == deletedQuestion);

            if (!ReferenceEquals(null, quizWithQuestion))
            {
                db.Entry(quizWithQuestion).State = EntityState.Deleted;                    
            }
        }
        db.SaveChanges();
    }

另一个尝试看起来像这样：

public void RemoveQuestionsFromQuiz(IEnumerable<int> deletedQuestions, int quizId)
{
    var quiz = // code which retrieves quiz

    foreach (var deletedQuestion in deletedQuestions)
    {
        var quizWithQuestion = quiz.QuizWithQuestions.FirstOrDefault(q => q.Id == deletedQuestion);

        if (!ReferenceEquals(null, quizWithQuestion))
        {
            foreach (var quizUserAnswer in quizWithQuestion.QuizUserAnswers)
            {
                quizUserAnswer.idQuizQuestion = null; // nullable
                quizWithQuestion.QuizUserAnswers.Remove(quizUserAnswer);
                db.Entry(quizUserAnswer).State = EntityState.Modified;
            }

            quiz.QuizWithQuestions.Remove(quizWithQuestion);

            db.Entry(quizWithQuestion).State = EntityState.Deleted;
        }
    }
    _db.SaveChanges();
}

我怎样才能删除这些该死的实体（我快要写一个存储过程了）？

Answer 1

由于您已经有了要删除的问题 ID，因此应该可以使用以下方法：

// assuming db is your DbContext
var questions = db.QuizWithQuestions
                  .Where(q => deletedQuestions.Contains(q.Id))
                  .Include(q => q.QuizUserAnswers);

// assuming this is your DbSet
db.QuizWithQuestions.RemoveRange(questions);

db.SaveChanges();

如果 QuizUserAnswer 实体被加载到上下文中（这是 include 应该做的），Entity Framework 应该处理将外键设置为 null。

Entity Framework 删除具有外键关系的实体时出错

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