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

db.collection.aggregate([
 {

  $group: {
   '_id':"$_id",
   'otherFields':{ $push: { fields: $ROOT } }
  } 
 }
])

Answer 2

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.

Answer 3

You should be using db.collection.find() rather than db.collection.aggregate():

db.collection.find().sort({"created":-1}).limit(1)

Answer 4

query = [
    {
        '$sort': {
            'created': -1
        }
    },
    {
        $group: { 
            '_id':null,
            'max':{'$first':"$$ROOT"}
        }
    }
]
db.collection.aggregate(query)

Answer 5

In the documentation i found that the $$ROOT expression addresses this problem.

From the DOC: http://docs.mongodb.org/manual/reference/operator/aggregation/group/#group-documents-by-author