Implement Followers


I’m doing an app where users can follow topics.
What is the best schema for implement with mongodb and meteor?

I thought to 2 solutions:

With one collection:
Schemas.Follow = new SimpleSchema({
userId: { type: String }
topicId: { type: String }

Pro: no problems with document 16MB limit
Cons: performances for search are slow (?)

use ids array in users and topics collection
Schemas.User = new SimpleSchema({

follows: { type: [String] }

Schemas.Topic = new SimpleSchema({

followedBy: { type: [String] }

Pro: better performance of search
Cons: problem of 16MB limit per document

Have you a better solution for mongodb and meteor?