MongoDB, Grouping by Multiple Fields - mongodb

Question

pretty new to Mongo and am finding some simple things that i would do in SQL frustratingly difficult in Mongo.
I have an object similar to this below
[{
"_id" : ObjectId("5870fb29a1fe030e1a2909db"),
"updatedAt" : ISODate("2017-01-07T14:28:57.224Z"),
"createdAt" : ISODate("2017-01-07T14:28:57.224Z"),
"state" : "Available",
},
{
"_id" : ObjectId("5870fb29a1fe030e1a2909dc"),
"updatedAt" : ISODate("2017-01-07T14:28:57.224Z"),
"createdAt" : ISODate("2017-01-07T14:28:57.224Z"),
"state" : "notReady",
},
{
"_id" : ObjectId("5870fb29a1fe030e1a2909d9"),
"updatedAt" : ISODate("2017-01-07T14:28:57.224Z"),
"createdAt" : ISODate("2017-01-07T14:28:57.224Z"),
"state" : "Disconnected",
}]
What i'm looking to do it group the data by the Maximum date and the state.
Ideally the result i would be looking for would be something like the following.
{
latestDate: "2017-01-07T14:28:57",
states : {
available : 10,
disconnected : 5,
notReady : 2
}}
Basically i'm looking for the SQL equivalent of this:
SELECT createdAt, state, COUNT(rowid)
FROM db
WHERE date = (SELECT MAX(createdAt) FROM db)
GROUP BY 1,2
I've searched around here and have found some good info but am probably missing something straight forward. Ive only managed to get here so far
db.collection.aggregate([
{$project: {"_id" : 0,"state": 1, "date" : "$createdAt"}},
{$group : {"_id" : {"date":"$date", "state": "actual"}, "count":{"$sum":1}}}
])
Any help would be appreciated :)

Answer 1

db.collection.aggregate([
{
$group : {
_id : {
date : "$createdAt",
state : "$state"
},
count : {$sum : 1}
}
},
{
$group : {
_id : "$_id.date",
states : {
$addToSet : {
state : "$_id.state",
count : "$count"
}
}
}
},
{
$sort : {_id : -1}
},
{
$limit : 1
},
{
$project : {
_id : 0,
latestDate : "$_id",
states : "$states"
}
}
])
output :
{
"latestDate" : ISODate("2017-01-07T14:28:57.224Z"),
"states" : [
{
"state" : "Available",
"count" : 1
},
{
"state" : "notReady",
"count" : 1
},
{
"state" : "Disconnected",
"count" : 1
}
]
}