MongoDB/Mongoose orderby date and groupby user - mongodb

Question

I have a collection where multiple documents may have the same userId field. I would like to groupby userId so that I get a list of unique userIds, but also a sort by date so that each returned document is the latest document for that userId. I've done queries like this with sql, and I'm really hoping its possible with mongo.
In this example collection:
{ userId: 456, date: 5/16/1988 },
{ userId: 456, date: 5/17/1988 },
{ userId: 789, date: 5/18/1988 },
{ userId: 789, date: 5/17/1988 }
I would want to return:
{ userId: 456, date: 5/17/1988 },
{ userId: 789, date: 5/18/1988 }

Answer 1

Here is how you would do it in mongo. Note that it got this to work with a date format of yyyy-mm-dd.
db.collection.aggregate({
$group: {
id : '$userId',
date: { $max: '$date'}
}
})
Sources: http://docs.mongodb.org/manual/

Answer 2

In your question you say you want the full document returned. You can do this by returning the full document as a field in the $group operator.
db.coll.aggregate([
{$sort:{date:-1}},
{$group: {_id: "$userId", doc:{$first: "$$CURRENT"}} }
])
I created four documents like in your question with Date type as some random dates. This would give you the following result:
{
"_id" : 789,
"doc" : {
"_id" : ObjectId("53d80e246ebc37d0c33321ba"),
"userId" : 789,
"date" : ISODate("2014-07-05T04:00:00.000Z")
}
},
{
"_id" : 456,
"doc" : {
"_id" : ObjectId("53d80e246ebc37d0c33321b8"),
"userId" : 456,
"date" : ISODate("2014-07-05T04:00:00.000Z")
}
}
See http://docs.mongodb.org/manual/reference/operator/aggregation/group/#variables for more info on $$CURENT

Answer 3

Although they probably are the right way to go, I wasn't able to use the db.collection.aggregate methods because I need to use other things like .populate() on a Model.find in this situation. So I came up with a work around where I sort on userID and date in the find(options) like so:
{ sort: { updateDate: -1, userId: -1 } }
Then I wrote a function on the front end to extract the latest record for each user:
filterLatest: function(docs) {
var lastUserId = null;
var latestDocs = [];
docs.forEach(function(doc) {
if(lastUserId != doc.userId) latestDocs.push(doc);
lastUserId = doc.userId;
});
return latestDocs;
}