how to use mongodb aggregate and retrieve entire documents

Question

I am seriously baffled by mongodb\'s aggregate function. All I want is to find the newest document in my collection. Let\'s say each record has a field \"created\"

Answer 1

I think I figured it out. For example, I have a collection containing an array of images (or pointers). Now I want to find the document with the most images

results=[];
db.collection.aggregate([
    {$unwind: "$images"},
    {$group:{_id:"$_id", 'imagecount':{$sum:1}}},
    {$group:{_id:"$_id",'max':{$max: "$imagecount"}}},
    {$sort:{max:-1}},
    {$group:{_id:0,'id':{$first:'$_id'},'max':{$first:"$max"}}}
]).result.forEach(function(d){
    results.push(db.stories.findOne({_id:d.id}));
});

now the final array will contain the document with the most images. Since images is an array, I use $unwind, I then group by document id and $sum:1, pipe that into a $group that finds the max, pipe it into reverse $sort for max and $group out the first result. Finally I fetchOne the document and push it into the results array.