public void addCourse(Course course) {
if (!courses.contains(course)) courses.add(course);
if (!course.getClients().contains(this)) course.getClients().add(this);
}
@AssertTrue(message = "course.full.exception")
public boolean isValid() {
int maxCapacity = course.getMaxiumumCapacity();
return registrations.size() <= maxCapacity;
}