The city director of law, when required to do so by resolution of the legislative authority of the city, shall prosecute or defend on behalf of the city, all complaints, suits, and controversies in which the city is a party, and such other suits, matters, and controversies as he is, by resolution or ordinance, directed to prosecute. He shall not be required to prosecute any action before the mayor of the city for the violation of an ordinance without first advising such action.