MYENTITY lRec = DbContext.MYENTITYs.First(x => x.TAG == name);
    lRec.TAG = "TEST";
    DbContext.SaveChanges();  <-- BOOM

modelBuilder.Entity<MYENTITY>().HasKey(t => t.DBINDEX);

public partial class MYENTITY
{
    public int DBINDEX { get; set; }
    public string TAG { get; set; }
    ...
}