In the quora.com index page, you can see if the answers are upvoted by you. It is easy to do this with relational database just using left join. But in parse/mongodb, you have to query one by one. To avoid too many queries, I cache the user's upvoted answers locally. Then we have new problem to sync the updated answers among multiple logins. Wondering if you have any better ideas?
It can be done in several ways. The simplest one, I think, is making two queries. One to get the posts to display, and another to get which post ids among these you upvoted previously, so that you can adjust their appearance. This, of course, implies that upvotes are stored in a separate collection.