$project to contain the latest date from the group - mongodb

Question

I have a collection that is filled with messages, and I want to get a group of how many messages there are per room.
Gets messages within the last hour
Where gameId = 5550d868c5242fb3299a2604
Grouped by $room
Sorted by created
When I add the projection, the count works, but _id becomes null, and created doesn't even return.
What I am looking for is _id to contain $room, created to contain the newest date, and count to be the count of items.
db.chatMessages.aggregate([
{
$match: {
created: {
$gte: new Date((new Date()) - (1000 * 60 * 60))
},
gameId: ObjectId("5550d868c5242fb3299a2604")
}
},
{
$project: {created: 1}
},
{
$group: {
_id: "$room",
count: {$sum: 1}
}
},
{
$sort: {created: -1}
}
])
Here is some example data:
{
"gameId" : ObjectId("5550d868c5242fb3299a2604"),
"message" : "For real?",
"room" : "Main",
"name" : "Hello",
"created" : ISODate("2015-05-24T17:49:43.868Z"),
"_id" : ObjectId("55620f376aa13616759f4f06")
}
{
"gameId" : ObjectId("5550d868c5242fb3299a2604"),
"message" : "Yeah for reals...",
"room" : "Main",
"name" : "Jim",
"created" : ISODate("2015-05-24T17:49:59.135Z"),
"_id" : ObjectId("55620f476aa13616759f4f07")
}
Here is the returned output:
{
"result" : [
{
"_id" : null,
"count" : 4
}
],
"ok" : 1
}
Here is what I am looking for:
{
"result" : [
{
"_id" : "Main",
"count" : 4,
"created" : ISODate("2015-05-24T17:37:19.331Z")
}
],
"ok" : 1
}
So what is causing this to break when I add the $project?

Answer 1

I was able to get it by adding created to the group and doing $max on the column like this:
db.chatMessages.aggregate([
{
$project: {room: 1, created: 1}
},
{
$match: {
created: {
$gte: new Date((new Date()) - (1000 * 60 * 60 * 20000))
},
gameId: ObjectId("5550d868c5242fb3299a2604")
}
},
{
$group: {
_id: "$room",
count: {$sum: 1},
created: {$max: "$created"}
}
},
{
$sort: {created: -1}
}
])