保存并编辑到数据库（实体框架）

Question

I have Like button, when it pressed - controller action should add field to my dbTable("Likes"), and if button pressed again - it should remove Like from dbTable("Likes").
Here code(simplified):

var like = db.Likes.Find(id);

if (like == null)
{
    Like like = new Like("SomeData", UserPosedLike);    
    db.Add(like);
}
else 
{
    db.Likes.Remove(like);
}

db.SaveChanges();

It's work fine at 95% of times, but if i press button very fast 2-3 times, it throws error – “Null ref”, some data in Like is NULL (but this data can't be null, because it references to other dbField(User)). Also, sometimes it adds two likes, but it shouldn't.

I think it happens because of:

First action(press) – read db, start processing.
Second action (second press) – read db, start processing. (first action not saved data yet).
First action(press) – save data.
Second action(pess) – save data.
Here we got 2 likes. Probably with “null error” similar problem.

I received advice to lock this block of code, but I think it's wrong. Also I thinking about Optimistic Concurrency, currently reading about it.

I'm not good with db, any help will be great - good book, code, article or advice. Thank you!

Answer 1

You need to read and write in transaction .

using (var db = new YourContext())
{
    using (var t = db.Database.BeginTransaction()) 
    {
        try
        {
            var like = db.Likes.Find(id);

            if (like == null)
            {
                Like like = new Like("SomeData", UserPosedLike);    
                db.Add(like);
            }
            else 
            {
                db.Likes.Remove(like);
            }

            db.SaveChanges();

            t.Commit();
        }
        catch
        {
            t.Rollback();
        }
    }
}

I received advice to lock this block of code, but I think it's wrong.

It is wrong.

Also as someone has suggested in comments, you can prevent users from clicking a button multiple times. You could disable the button and add a spinner.