前面的on 是on the farm,是主语中的on
后面关系词填什么,看它在定语从句中的成分.这里的定语从句是we worked on the farm,是做状语,所以用where.如果是It‘s on the farm which we worked on ,就用which；或者It's on the farm on which we worked.你的句子似乎变为we worked on on the farm,我认为原句如果是It‘sthe farm （）we worked，才能用wherewe worked on是定语，只能放在the farm后。这个句子的主语 是It's on the farm。而不是你写的句子语序。
完整的翻译是：它在我们工作过的农场里。那请问有没有on the farm which we worked没有 which 要改where
如果是 on the farm which we visited 就可以，因为farm做visited的宾语。 也就是visited the farm
先行词做定语从句的宾语或主语，用关系代词which、who等；先行词做定语从句的状语，用关系副词when， where等