MongoDB sorting and limit inside an array

Question

I have a collection of conversations like this:

    [
        {
            title: ...,
            members: [...],
            messages: [
                { _id: ..., date: ..., text: ... },
                { _id: ..., date: ..., text: ... },
                ...
            ]
        },
        {
            title: ...,
            members: [...],
            messages: []
        },
        ...
    ]

Some documents has an empty messages array. I need to select a several collections (for one user), but two things:

1) The order of messages must be reversed (or messages must be ordered by date desc, result is the same).
2) The number of messages for each conversation must be limited.

I tried to reverse messages using this query:

    db.conversations.aggregate([
        {$match: {members: 'someUserId'}}
        {$unwind: '$messages'},
        {$sort: {'messages.date': -1}},
        {$group: {
            _id: '$_id',
            title: {$first: '$title'},
            members: {$first: '$members'},
            messages: {$push: '$messages'}
        }}
    ])

But there is two problems again: conversation with no messages are not selected and messages still not limited.

Do you have any ideas, how to solve it? Or how to solve one of this problems? Thanks!

Answer 1

There is a sort and slice operator that can be used in conjunction when you push the messages into the document arrays:

http://docs.mongodb.org/manual/reference/operator/sort/ http://docs.mongodb.org/manual/reference/operator/slice/

They keep your array sorted and limited respectively.