将 SaveChanges() 方法与 EF 核心一起使用时出错

Question

In the following code, I receive an error in _context.SaveChanges(); when adding a new record to FeedbackComments inside the first foreach loop. The error is New transaction is not allowed because there are other threads running in the session. . Any ideas why this is happening?

BTW, I keep receiving the same error when SaveChanges is called only once after the outer loop.

List<FeedbackComment> feedbackComments = comments.Select(c => new FeedbackComment
{
    Id = c.Id,
    CommentText = c.Content,
    SubmissionId = submissionId,
    UserDisplayName = c.Author.DisplayName,
    DateCreated = c.CreatedTime.GetValueOrDefault(),
    FeedbackReplies = c.Replies.Select(r => new FeedbackReply
    {
        Id = r.Id,
        UserDisplayName = r.Author.DisplayName,
        ReplyText = r.Content,
        DateCreated = r.CreatedTime.GetValueOrDefault(),
        FeedbackCommentId = c.Id
    }).ToList()
}).ToList();

_context.SaveChanges();


foreach (FeedbackComment c in feedbackComments)
{
    if (!_context.FeedbackComments.Any(fc => fc.Id == c.Id))
    {
        ApplicationUser commentOwner = _context.ApplicationUsers.FirstOrDefault(au => au.GoogleDisplayName == c.UserDisplayName);

        if(commentOwner != null)
        {
            c.UserId = commentOwner.Id;

            _context.FeedbackComments.Add(c);
            newComments = true;
            _context.SaveChanges();

        }

    }


    foreach (FeedbackReply r in c.FeedbackReplies)
    {
        if (!_context.FeedbackReplies.Any(fr => fr.Id == r.Id))
        {
            ApplicationUser replyOwner = _context.ApplicationUsers.FirstOrDefault(au => au.GoogleDisplayName == c.UserDisplayName);

            if (replyOwner != null)
            {
                r.UserId = replyOwner.Id;

                _context.FeedbackReplies.Add(r);
                newComments = true;
                _context.SaveChanges();

            }
        }

    }
}

Answer 1

When you are trying to save a change using a transaction, you should wait until any other transactions are completed. On the other hand, waiting for the previous transaction to be completed, makes severe performance issue. You should put _context.SaveChanges(); outside the foreach loop like this:

foreach (FeedbackReply r in c.FeedbackReplies)
{
    if (!_context.FeedbackReplies.Any(fr => fr.Id == r.Id))
    {
        ApplicationUser replyOwner = _context.ApplicationUsers.FirstOrDefault(au => au.GoogleDisplayName == c.UserDisplayName);

        if (replyOwner != null)
        {
            r.UserId = replyOwner.Id;
            _context.FeedbackReplies.Add(r);
            newComments = true;
        }
    }
}
_context.SaveChanges();

In the above code, all changes are applied to the database in one transaction.

Answer 2

Why are you calling SaveChanges after the initial data fetch? If you leave that one out and call SaveChanges only once, at the end, it should work, as Hadi said.