MongoDB One of Each

Question

I'd like to retrieve a mongo selector for the latest posts for each of the catagory ids I specify.

Here is an example of objects in the forumTopics collection:

{
    _id: ...,
    createdTime: [unix epoch timestamp],
    catagory: "someid"
}

In my code, I have an array of the category IDs I would like:

catagories = ["someid", "someotherid"]

I can fetch the posts for the catagories like this:

forumTopics.find {catagory: {$in: catids}}

My question is how I can fetch just one topic object for each category, the one fetched object being the one with the greatest createdTime. I know how to fetch with a limit of 1, but I'm not sure how to get one for each category in the $in.

Answer 1

You can do this by using the aggregation framework:

forumTopics.aggregate( [
    { $match: { catagory: {$in: catids} } },
    { $sort: { createdTime: 1 } },
    { $group: {
            _id: "$catagory",
            forumTopicId: {
                $last: "$_id"
            }
        }
    }
] )