how to use mongodb aggregate and retrieve entire documents - mongodb

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"
db.collection.aggregate({
$group: {
_id:0,
'id':{$first:"$_id"},
'max':{$max:"$created"}
}
})
yields the correct result, but I want the entire document in the result? How would I do that?
This is the structure of the document:
{
"_id" : ObjectId("52310da847cf343c8c000093"),
"created" : 1389073358,
"image" : ObjectId("52cb93dd47cf348786d63af2"),
"images" : [
ObjectId("52cb93dd47cf348786d63af2"),
ObjectId("52f67c8447cf343509d63af2")
],
"organization" : ObjectId("522949d347cf3402c3000001"),
"published" : 1392601521,
"status" : "PUBLISHED",
"tags" : [ ],
"updated" : 1392601521,
"user_id" : ObjectId("52214ce847cf344902000000")
}

Answer 1

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

Answer 2

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

Answer 3

db.collection.aggregate([
{
$group: {
'_id':"$_id",
'otherFields':{ $push: { fields: $ROOT } }
}
}
])

Answer 4

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 5

You should be using db.collection.find() rather than db.collection.aggregate():
db.collection.find().sort({"created":-1}).limit(1)