Return whole document from aggregation - mongodb

Question

I'm using the following query to fetch one most recent comment for every post in database:
db.comments.aggregate([
{
"$match": {
"post_id": {
"$in": [ObjectId("52c5ce24dca32d32740c1435"), ObjectId("52c5ce24dca32d32740c15ad")]
}
}
},
{
"$sort": {"_id": -1}
},
{
"$group": {
"_id": "$post_id",
"lastComment": {
"$first": "$_id"
}
}
}
])
I expect it to return the whole comment's document but it only returns the _id field of each document. So what would be the proper way to get all most recent comments as a whole document (or at least include some other fields)?

Answer 1

Currently you cannot get the whole comment document via single $first operator. But you can include other necessary fields (similar to _id field) during $group step:
{
"$group": {
_id: "$post_id",
lastComment: { "$first": "$_id" },
field_1: { "$first": "$field_1" },
field_2: { "$first": "$field_2" },
// ...
field_N: { "$first": "$field_N" }
}
}
According to this JIRA ticket: https://jira.mongodb.org/browse/SERVER-5916, the whole document will be available to return from aggregation operations from 2.5.3 version. It will be possible using new variables: $$ROOT or $$CURRENT:
{
"$group": {
_id: "$post_id",
lastComment: { "$first": "$$CURRENT" }
}
}

Answer 2

As suggested, we can do :
{
"$group": {
_id: "$post_id",
lastComment: { "$first": "$$CURRENT" }
}
}
and then do use { '$replaceRoot': { 'newRoot': '$lastComment' } } on any mongodb server 3.4 or above to unwrap object from {lastComment:{actualEntireObj}},{lastComment:{actualEntireObj}} to {},{} this way it will get embedded $$ROOT document to the top level and replaces all other fields like _id returning from $group stage of aggregation.
db.collection.aggregate([
{
"$match": {
"post_id": {
"$in": [ObjectId("52c5ce24dca32d32740c1435"), ObjectId("52c5ce24dca32d32740c15ad")]
}
}
},
{
"$sort": { "_id": -1 }
},
{
"$group": {
_id: "$post_id",
lastComment: { "$first": "$$CURRENT" }
}
},
{ '$replaceRoot': { 'newRoot': '$lastComment' } }
])