ASP.NET Core MVC + Entity Framework Core 如何获取新增对象的id

Question

我有以下操作方法来添加父 (Submission) 和子 (SubmissionQuestionSubmission) 对象：

public async Task<IActionResult> Create([Bind("Submission,SubmissionQuestionSubmission")] SubmissionCreate sc )
{
    if (ModelState.IsValid)
    {
        var newsubmission  =  _context.Submission.Add(sc.Submission);
        sc.Submission.Created = DateTime.Now;
        await _context.SaveChangesAsync();

        foreach (var v in sc.SubmissionQuestionSubmission)
        {
            v.SubmissionId = sc.Submission.Id;//i though this will be null..
            _context.SubmissionQuestionSubmission.Add(v);
        }

        await _context.SaveChangesAsync();
    }
}

一开始我以为代码行不通，因为sc.Submission.Id 父外键将为空，因为我刚刚添加了提交对象。但是我的代码运行良好，那么 ASP.NET Core MVC 和 Entity Framework Core 是如何检索到新添加对象的Id 的呢？它会自动往返于数据库吗？

这是我的模型类：

public partial class Submission
{
    public Submission()
    {
        SubmissionQuestionSubmission = new HashSet<SubmissionQuestionSubmission>();
    }

    public long Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime? Created { get; set; }

    public virtual ICollection<SubmissionQuestionSubmission> SubmissionQuestionSubmission { get; set; }
}

public partial class SubmissionQuestionSubmission
{
        public int SubmissionQuestionId { get; set; }
        public long SubmissionId { get; set; }
        public bool? Answer { get; set; }

        public virtual Submission Submission { get; set; }
        public virtual SubmissionQuestion SubmissionQuestion { get; set; }
}

Answer 1

当调用SaveChanges() 时，它将自动更新新保存实体的Id 字段。例如，EF 在保存新实体时可能会生成如下查询：

SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ
BEGIN

INSERT INTO `Author` (`Name`)
VALUES ('test');

SELECT `Id`
FROM `Author`
WHERE ROW_COUNT() = 1
 AND `Id`=LAST_INSERT_ID()

COMMIT

如您所见，它将运行INSERT INTO 查询，然后运行SELECT 查询以获取新插入行的Id，该行将保存在Id 属性中。