AddGrade erase any previous grade recorded for the user

works.go

@@ -472,6 +472,19 @@ func (u *User) GetMyWorkGrade(w *Work) (g WorkGrade, err error) {
 }
 
 func (w *Work) AddGrade(grade WorkGrade) error {
+	u := User{Id: grade.IdUser}
+
+	// Search a previous record
+	g, err := u.GetMyWorkGrade(w)
+	if err != nil && err != sql.ErrNoRows {
+		return err
+	} else if err == nil {
+		_, err = g.Delete()
+		if err != nil {
+			return err
+		}
+	}
+
 	return w.AddGrades([]WorkGrade{grade})
 }