When declaratory relief is sought, all persons shall be made parties who have or claim any interest which would be affected by the declaration, and no declaration shall prejudice the rights of persons not parties to the proceedings. In any proceeding which involves the validity of a municipal ordinance or franchise, such municipality shall be made a party, and shall be entitled to be heard, and if the statute, ordinance or franchise is alleged to be unconstitutional, the Attorney General of the State shall also be served with a copy of the proceeding and be entitled to be heard. (1931, c. 102, s. 8.)