如何异步获取和选择真正正确的最新数据

Question

In a website, there are an article page and when i go to the article page, it displays the article and the first 10 comments belong to this article.

And there are some button when click it, it asynchronously display the rest of all comments by AJAX.

The question is that suppose after the article page is loaded with first 10 comments, User that own comment or Admin delete it, how to select the rest of comments? if i skip first 10 comments from select statement, there are some comments not selected.

As Example:

suppose the comments IDs are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20

First 10 comments IDs i select are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

IEnumerable<Comment> comments = (from a in context.Comments
                                         where a.ArticleID == ArticleID
                                         orderby a.CommentDate descending
                                         select new Comment
                                         {
                                             CommentID = a.CommentID,
                                             CommentContent = a.CommentContent,
                                             CommentDate = a.CommentDate,


                                         }).ToList().Take(10);

suppose User that own comment or Admin delete 2 comments whose IDs: 9, 10

then the first ten comments in databse will become: 1, 2, 3, 4, 5, 6, 7, 8, 11, 12

when i try to get the rest of comments and skip first 10 comments, then the result is: 13, 14, 15, 16, 17, 18, 19, 20

IEnumerable<Comment> comments = (from a in context.Comments
                                         where a.ArticleID == ArticleID
                                         orderby a.CommentDate descending
                                         select new Comment
                                         {
                                             CommentID = a.CommentID,
                                             CommentContent = a.CommentContent,
                                             CommentDate = a.CommentDate,


                                         }).ToList().Skip(10);

so there are two comments 11 & 12 i cannot see them, because comments with IDs: 11, 12 will be consider with the first 10 comments...

Answer 1

You need to use Skip and Take together in the same LINQ query, and the values passed to those functions should be dynamic based on what the data that was already presented in the application.

So, for the initial page display, you would have a query like this:

var toSkip = 0; var toTake = 10;

IEnumerable<Comment> comments = (from a in context.Comments
    where a.ArticleID == ArticleID
    orderby a.CommentDate descending
    select new Comment
    {
        CommentID = a.CommentID,
        CommentContent = a.CommentContent,
        CommentDate = a.CommentDate
    })
    .Skip(toSkip).Take(toTake).ToList();

Subsequent queries would change the values in the toSkip and toTake variables appropriately. Ideally , these would be parameters sent into the MVC action that is used to get the data the application needs.

With this approach, you will need to have a way to tell how many to skip based on the data left from the prior, or initial, list. A LINQ query to get the count of items based on the Min and Max comment Id from the current list should allow you to derive the number to skip. This query would take into consideration the data that was delete between requests, too. It's a seemingly extra query, so I am not that excited about that.

You could use the MAX comment Id from the prior list, and filter like this.

var toTake = 10; var maxCommentId = 10;

IEnumerable<Comment> comments = (from a in context.Comments
    where a.ArticleID == ArticleID
    && a.CommentID > maxCommentId // <- new filter here
    orderby a.CommentDate descending
    select new Comment
    {
        CommentID = a.CommentID,
        CommentContent = a.CommentContent,
        CommentDate = a.CommentDate
    })
    .Take(toTake).ToList();

Again, the variable values would need to change based on the current application state; making them action parameters being ideal. Something like this:

public async IActionResult GetComments(int articleID,
  int toSkip = 0,
  int toTake = 10)
    {
        IEnumerable<Comment> comments = (from a in context.Comments
          where a.ArticleID == articleID
          orderby a.CommentDate descending
          select new Comment
          {
              CommentID = a.CommentID,
              CommentContent = a.CommentContent,
              CommentDate = a.CommentDate
          })
          .Skip(toSkip).Take(toTake).ToList();
        return comments;
    }

This is not a full solution, but it should give you an idea of what direction to take that works best for your application. I hope it helps you out.