MongoDB Structure for message app

I see that this question is old, but for anyone interested, a similar question was asked and one answer looks viable https://stackoverflow.com/a/30830429/132610

Conversation : {
 id: 123,
 members: [ user_id1, user_id2 ]
}
Message { conversationId: 123, author: user_2, body: 'Hi what's up' }
Message { conversationId: 123, author: user_1, body: 'Whanna ask some question on stackoverflow' }

Update #1

1) Scalability: MongoDB scales well with very large collection. Billions of messages per collection. There is a technique called sharding that can allow you to split larger collection to multiple nodes.

2) Reading. Since MongoDB has indexing mechanisms, reads are comparable to any fine-tuned database engine. So reading will not be an issue. Especially, when a conversation(group|room) has fewer participants, for example two people messaging each other.