Limit Multiple MongoDB Array Size

Question

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

Answer 1

If I understand correctly Capped Collections are what you want. http://www.mongodb.org/display/DOCS/Capped+Collections

Answer 2

As it turns out, this was a longstanding issue in MongoDB that was since added in MongoDB 2.4 release using the $slice operator.

db.students.update(
                { _id: 1 },
                { $push: { scores: { $each : [
                                               { attempt: 3, score: 7 },
                                               { attempt: 4, score: 4 }
                                             ],
                                     $sort: { score: 1 },
                                     $slice: -3
                                   }
                          }
                }
              )

http://docs.mongodb.org/manual/tutorial/limit-number-of-elements-in-updated-array/