Has Many Through query.

I have three models : topic, class and enrollment

Topic :

1. id
2. name

class :

1. id
2. topic_id
3. name

enrollment :

1. id
2. class_id

I want to query enrollment model, to get the details of topic associated with each class. I am using hasManyThrough relation like following:

return $this->hasManyThrough('Topic', 'Class', 'topic_id', 'id', 'class_id', 'id');

but its not working. Please help. Thanks