Mariana Victoria of Spain ( 31 March 1718 â `` 15 January 1781 ) sometimes Maria Anna Victoria was an Infanta of Spain by birth and was later the Queen of Portugal as wife of King Joseph I. The eldest daughter of Philip V of Spain and Elisabeth Farnese , she was engaged to the young Louis XV of France at the age of seven . Mariana Victoria of Spain ( 31 March 1718 â `` 15 January 1781 ) was an Infanta of Spain by birth and was later the Queen of Portugal as wife of King Joseph I. The eldest daughter of Philip V of Spain and Elisabeth Farnese , she was engaged to the young Louis XV of France at the age of seven .