  We construct models where initial and boundary conditions can be found from
the fundamental rules of physics, without the need to assume them, they will be
derived from the action principle. Those constraints are established from
physical view point, and it is not in the form of Lagrange multipliers. We show
some examples from the past and some new examples which can be useful, where
constraint can be obtained from the action principle. Those actions represent
physical models. We show that it is possible to use our rule to get those
constraints directly.
