Limit Multiple MongoDB Array Size
===

I have a document that lists post item ids for an author separated out by topic. This results in a document such as the following:

    {
        _id: "sdkafjsadkfjads3023",
        Author: "SomeGuy"
        RecentPosts: {
            "topic-1": {
                Count: 4,
                Posts: ["postitemid1","postitemid2","postitemid2","postitemid3"]
            }
            "topic-2": {
                Count: 3
                Posts: ["postitem5","postitem6","postitem8"]
            }
        }
    }

Most of the time I am doing atomic pushes to each of these post arrays in the same update. What I want to do is limit the arrays above to 10 items at all times. This way, anytime I do a pushall to the same topic/posts. Is what I'm asking even possible, or should I do this a different way?

Thanks in advance