Detect if linq entity needs inserting

Question

Usually I have an integer identity primary key I can detect if an object needs to be inserted by doing

public partial class MyObject
{
    public void Save(DataContext context)
    {
        if (this.PrimaryKey == 0) context.MyObjects.InsertOnSubmit(this);
        context.SubmitChanges();
    }
}

But in the case I'm working on the primary key is a string and is populated by code. Is there any way to create a similar save method that inserts the object automatically if necessary? Thanks.