hobby-horse
n
1 long stick with a horse´s head, used as a toy 马头杆子(玩具); 竹马.
2 subject that a person likes to discuss; favourite topic of conversation (讨论、 交谈中)喜爱的话题
You´ve got me onto (ie talking about) one of my favourite hobby-horses. 你把我引到我喜爱的一个话题上来了.