return document with latest subdocument only in mongodb aggregate

Question

I\'ve got these Mongoose Schemas:

var Thread = new Schema({
    title: String, messages: [Message]
});
var Message = new Schema({
    date_added: Date, auth         


        

Answer 1

You can use $unwind, $sort, and $group to do this using something like:

Thread.aggregate([
    // Duplicate the docs, one per messages element.
    {$unwind: '$messages'}, 
    // Sort the pipeline to bring the most recent message to the front
    {$sort: {'messages.date_added': -1}}, 
    // Group by _id+title, taking the first (most recent) message per group
    {$group: {
        _id: { _id: '$_id', title: '$title' }, 
        message: {$first: '$messages'}
    }},
    // Reshape the document back into the original style
    {$project: {_id: '$_id._id', title: '$_id.title', message: 1}}
]);