MongoDB aggregate Query with select fields - mongodb

Question

I want to make chat system, and also need get last message of user which aggigation. I also provide query with this but it only return userId of user. so please help me, thanks
Database:
/* 1 */
{
"_id" : ObjectId("56937df0418a6afab248616d"),
"to" : ObjectId("56728051d4b426be03de18f2"),
"from" : ObjectId("568e402eaecfa53282f60d17"),
"msg" : "Hello!",
"cd" : ISODate("2016-01-11T10:03:28.139Z"),
"type" : "other",
"ir" : 0
}
/* 2 */
{
"_id" : ObjectId("56937e01418a6afab248616e"),
"to" : ObjectId("568e402eaecfa53282f60d17"),
"from" : ObjectId("56728051d4b426be03de18f2"),
"msg" : "Hi!",
"cd" : ISODate("2016-01-11T10:03:45.588Z"),
"type" : "other",
"ir" : 0
}
/* 3 */
{
"_id" : ObjectId("56937e45418a6afab248616f"),
"to" : ObjectId("56728051d4b426be03de18f2"),
"from" : ObjectId("568e402eaecfa53282f60d17"),
"msg" : "Shu che ela!",
"cd" : ISODate("2016-01-11T10:04:53.280Z"),
"type" : "other",
"ir" : 0
}
Query:
db.getCollection('chat_message').aggregate( [
{
$match: {
ir: 0,
$or : [
{"to" : ObjectId("56728051d4b426be03de18f2")}
]
}
},
{ $group: { _id: "$from" } },
])
I Run this query but not get result which i want
Require Out came:
/* 1 */
{
"result" : [
{
"_id" : ObjectId("568e402eaecfa53282f60d17"),
"msg" : "Shu che ela!"
}
],
"ok" : 1.0000000000000000
}

Answer 1

You are right track but you missing some things about $mongo object Id
In MongoDB, documents stored in a collection require a unique _id field that acts as a primary key.
so when you run your aggregation query $group creates _id for from key but in your document structure looks like from having two documents with same ObjectId since the result return first matching criteria. This return only "msg" : "Hello!", or "msg" : "Shu che ela!" which documents inserted first return first.
so should changed your aggreation like this {"$group":{"_id":"$_id","msg":{"$first":"$msg"}}} you will get both documents.