多對多的關係 - 我做對了嗎？

Question

這個想法很簡單。我有一個標籤列表。當我創建一個問題時，我想添加一些標籤。

型號：

public class QuestionModel 
{ 
    public int Id { get; set; } 

    public String Content { get; set; } 

    public ICollection<TagModeltoQuestionModel> Tags { get; set; } 

    [NotMapped] 
    public ICollection<TagModel> AssignedTags { get { return Tags.Select(x => x.Tag).ToList(); } } 

    public int UserId { get; set; } 
} 

public class QuestionViewModel // helper - not in database 
{ 
    public QuestionModel Model { get; set; } 
    public ICollection<TagModel> Tags { get; set; } 
} 

public class TagModel 
{ 
    public int Id { get; set; } 

    public String Name { get; set; } 

    public ICollection<TagModeltoQuestionModel> Questions { get; set; } 

    [NotMapped] 
    public bool Assigned { get; set; } 

    [NotMapped] 
    public ICollection<QuestionModel> AssignedQuestions { get { return Questions.Select(x => x.Question).ToList(); } } 

} 

public class TagModeltoQuestionModel // many to many 
{ 
    [Key, Column(Order = 0)] 
    public int TagId { get; set; } 
    [Key, Column(Order = 1)] 
    public int QuestionId { get; set; } 

    public virtual QuestionModel Question { get; set; } 
    public virtual TagModel Tag { get; set; } 
} 

控制器：

[HttpPost] 
public ActionResult Edit(QuestionViewModel questionViewModel) 
{ 
    if (ModelState.IsValid) 
    { 
     _repo.Update(questionViewModel.Model, questionViewModel.Tags); // see repo code below 
     return RedirectToAction("Index"); 
    } 
    return View(questionViewModel.Model); 
} 

回購：

public void Update(QuestionModel entity, ICollection<TagModel> tags) 
{ 
    AssignTags(entity, tags); 
    Db.Attach(entity); 
    Db.SaveChanges(); 
} 

private void AssignTags(QuestionModel entity, ICollection<TagModel> tags) 
{ 
    tags = tags.Where(x => x.Assigned).ToArray(); // remove unassigned comming form View --> Controller 

    var linkedTags = 
     Db.TagsToQuestions.Where(x => x.QuestionId == entity.Id); 
    var linkedTagsIds = linkedTags.Select(x => x.TagId); 

    var selectedTagsIds = tags.Select(x => x.Id); 
    var oldTags = linkedTags.Where(x => !selectedTagsIds.Contains(x.TagId)); 
    var newTags = tags.Where(x => !linkedTagsIds.Contains(x.Id)).Select(x=> new TagModeltoQuestionModel{QuestionId=entity.Id,TagId=x.Id}); 

    foreach (var t in oldTags) 
     Db.Delete(t); 

    foreach (var t in newTags) 
     Db.Add(t); 

    Db.SaveChanges(); 
} 

這工作得很好，雖然我不知道這是去（正道實際上我自己實現了整個多對多邏輯）。有沒有更聰明的方法讓EF爲我做這份工作？我通過一堆教程挖掘，但沒有一個爲我工作。

此外，我覺得AssignTags方法可以寫得更好，所以任何意見也表示讚賞。

編輯

根據haim770的回答我的簡化模型，他建議的方式。

我的控制器現在看起來像這樣：

public void Update(QuestionModel entity, ICollection<TagModel> tags) 
{ 
    Db.Attach(entity); 

    //these lines give the same result 
    //var ids = tags.Select(y => y.Id).ToArray(); 
    //entity.Tags = Db.Tags.Where(x => ids.Contains(x.Id)).ToArray(); 

    tags.ForEach(x => Db.Attach(x)); 
    entity.Tags = tags; 
    Db.SaveChanges(); 
} 

的SaveChanges導致錯誤：

An error occurred while saving entities that do not expose foreign key properties for their relationships. The EntityEntries property will return null because a single entity cannot be identified as the source of the exception. Handling of exceptions while saving can be made easier by exposing foreign key properties in your entity types. See the InnerException for details. 
inner: 
{"A duplicate value cannot be inserted into a unique index. [ Table name = TagModelQuestionModels,Constraint name = PK_TagModelQuestionModels ] 

那麼如何正確地執行呢？

Answer 1

你不需要TagModeltoQuestionModel類。你可以模擬many-to-many關係是這樣的：

public class QuestionModel 
{ 
    //.... 
    public ICollection<TagModel> Tags { get; set; } 
} 

public class TagModel 
{ 
    //.... 
    public ICollection<QuestionModel> Questions { get; set; } 
} 

Question擁有許多Tags參考，每個Tag擁有許多Questions參考。

的Entity Framework整點（像任何其他ORM）是不必你的對象和他們的關係模型在database-like的方式，而是讓你它在那麼一個純粹的Object Oriented方式讓ORM模型饒你的「骯髒的工作'的中間表，外鍵等...