Question

I currently have an object like this (simplified):

public class Image {
    public int Id { get; set; }

    public int ExternalId { get; set; }
}

Now let’s say I have this method (mostly pseudo-code):

public void GetImage(int externalId) {
    var existingImage = db.Images.FirstOrDefault(i => i.ExternalId == externalId);

    if (existingImage != null) {
        return existingImage;
    }

    var newImage = new Image() { ExternalId = externalId };
    db.Images.Attach(newImage);
    db.SaveChanges();

    return newImage;
}

Because ExternalId isn’t a key, the change tracker won’t care if I have “duplicate” images in the tracker.

So now, let’s say this method gets called twice, at the same time via AJAX and Web API (my current scenario). It’s async, so there are two threads calling this method now.

If the time between calls is short enough (in my case it is), two rows will be added to the database with the same external ID because neither existing check will return a row. I’ve greatly simplified this example, since in my real one, there’s a timing issue as I fetch the “image” from a service.

How can I prevent this? I need the image to be returned regardless if it’s new or updated. I’ve added a Unique Constraint in the database, so I get an exception, but then on the client, the call fails whereas it should use the existing image instead of throwing an exception.

If I understand EF correctly, I could handle this by making ExternalId a primary key and then use concurrency to handle this, right? Is there any way to avoid changing my current model or is this the only option?

Answer 1

If you already have property defining uniqueness of your entity (ExternalId) you should use it as a key instead of creating another dummy key which does not specify a real uniqueness of your entity. If you don’t use ExternalId as a key you must put unique constraint on that column in the database and handle exception in your code to load existing Image from the database.