c# - 是什么原因导致 INSERT 语句与 FOREIGN KEY 约束冲突？

Question

这段代码以前对我有用，但我不确定是什么导致了这个错误。我唯一的猜测是，当我尝试创建一个玩家时，团队数据被发送回团队表并试图复制，但由于 TeamId 是唯一的，因此出现此错误。

错误

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_dbo.Players_dbo.Teams_TeamId". The conflict occurred in database "Web", table "dbo.Teams", column 'TeamId'. The statement has been terminated.

播放器

public class Player
{
    ...
    ...

    [HiddenInput(DisplayValue = false)]
    [ForeignKey("Team")]
    public int TeamId { get; set; }        

    public virtual Team Team { get; set; }

    ...
}

团队

public class Team
{
    [Key]
    [HiddenInput(DisplayValue = false)]
    public int TeamId { get; set; }

    ....

    public virtual ICollection<Player> Players { get; set; }
}

Controller

//
    // GET: /Player/
    [HttpGet]
    public ActionResult Create()
    {
        PopulateTeamsDropDownList();
        return View();
    }

    //
    // POST: /Player/
    [HttpPost]
    public ActionResult Create(Player model)
    {
        if (ModelState.IsValid)
        {
            using (var db = new EfDb())
            {
                var userProfile = db.UserProfiles.Single(u => u.UserName == User.Identity.Name);
                if (userProfile != null)
                {
                    var player = new Player
                                     {
                                         UserProfile = userProfile,                                             
                                         ....
                                         ....                                              
                                     };                        
                    db.Players.Add(player);                          

                    db.SaveChanges();
                }
            }
        }
        PopulateTeamsDropDownList(model.TeamId);
        return View(model);
    }

    private void PopulateTeamsDropDownList(object selectedTeams = null)
    {
        var teamsQuery = from d in _iService.Teams
                         orderby d.Name
                         select d;
        ViewBag.TeamID = new SelectList(teamsQuery, "TeamId", "Name", selectedTeams);
    }

查看

<div class="editor-label">
            @Html.LabelFor(model => model.TeamId, "Pick Your Team")
    </div>
    <div class="editor-field">
        @Html.DropDownList("TeamId", String.Empty)
        @Html.ValidationMessageFor(model => model.TeamId)
    </div>

如何解决这个错误？

Answer 1

这背后的原因是，当您在 child 表中插入一条记录时，该引用列的值在 parent 表中尚不存在.

考虑以下场景:

// PARENT TABLE
CREATE TABLE TableA
(
   ID INT PRIMARY KEY
);

// CHILD TABLE
CREATE TABLE TableB
(
   ID INT PRIMARY KEY,
   TableA_ID INT,
   CONSTRAINT tb_FK FOREIGN KEY (TableA_ID) REFERENCES TableA(ID)
);


// RECORDS OF PARENT TABLE
INSERT INTO TableA (ID) VALUES (1);
INSERT INTO TableA (ID) VALUES (2);

// RECORDS OF CHILD TABLE
INSERT INTO TableB (ID, TableA_ID) VALUES (1,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (2,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (3,2);

如果执行上面的语句，它不会失败，因为它们都没有违反引用完整性规则。

尝试执行以下语句:

INSERT INTO TableB (ID, TableA_ID) VALUES (3,4);

失败是因为 Table1.ID 上不存在要插入的 TableA_ID 的值 4。外键保留了记录之间的参照完整性。