Preventing duplicate inserts into database

Question

I have a page of announcements that when a user views it, adds it to a "seen" database. The issue with this is, if I user then clicks the same page twice then it records that twice and so on. Here is my code, How could I check if the entry already exists in the database?

public ActionResult Index()
{
    string currentUserId = User.Identity.GetUserId();
    var currentUser = db.Users.FirstOrDefault(x => x.Id == currentUserId);

    List<Seen> seens = new List<Seen>();  

    foreach (Announcement anoun in db.Announcements)
    {
        seens.Add(new Seen
                      {
                          User = currentUser, // You have this already so why go to the database again?
                          Announcement = anoun, // Same with this.
                      });
    }

    db.Seens.AddRange(seens);
    db.SaveChanges();

    return View(db.Announcements.ToList());
}

You just add to the DB, where is your code to check if it already exists in DB? — L.B, Dec 16 '16 at 21:27

score 1 · Answer 1 · edited May 23 '17 at 12:00

There are several things to consider:

1) database integrity - consider adding UNIQUE constraints as a safety net for business uniqueness - in your case (SeenId, UserId) or something similar. This will also add an index that makes some selections faster.

2) prevent double submit - first point prevents persistence of duplicates, but will also lead to error. In order to avoid this, you should take a look upon preventing double submission.

In MVC, a question dealing with preventing double submission shows how Post/Redirect/Get Pattern is implemented in MVC.

Sina M.Azad · Answer 2 · 2016-12-17T03:29:16.077

How could I check if the entry already exists in the database?

well , you could simply set your SQL field to Unique to prevent multi entry of data , As I assume that you are working on a social network type of app and you want each user to be able to "view" your post/page only once.

another way is to use the same loop to search for the value in the field , and if it is there then it it just quits the process.

score 0 · Answer 3 · answered Dec 16 '16 at 22:05

There is some ways you can prevent it.

Set your value as primary key in your database/table (uniqueness)
Check for existing value in your code behind before inserting into DB (redirect or return message if exist)
Disable submission button in client side page just after first click

Preventing duplicate inserts into database

3 Answers3