Group by reduced field depending on variable in mongodb - mongodb

Question

I have the following collection for messages:
{
"_id" : ObjectId("56214d5632001bae07a6e6b3"),
"sender_id" : 8,
"receiver_id" : 2,
"content" : "fdgfd",
"state" : 1,
"timestamp" : 1445023062899.0000000000000000
},
{
"_id" : ObjectId("56214d5c32001bae07a6e6b4"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "fasfa",
"state" : 1,
"timestamp" : 1445023068443.0000000000000000
},
{
"_id" : ObjectId("56214d8032001bae07a6e6b5"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "dfdsfds",
"state" : 1,
"timestamp" : 1445023104363.0000000000000000
},
{
"_id" : ObjectId("56214d8032001bae07a6e6b6"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "fdsf",
"state" : 1,
"timestamp" : 1445023104825.0000000000000000
},
{
"_id" : ObjectId("56214d8132001bae07a6e6b7"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "sfsdfs",
"state" : 1,
"timestamp" : 1445023105436.0000000000000000
},
{
"_id" : ObjectId("56214d8132001bae07a6e6b8"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "f",
"state" : 1,
"timestamp" : 1445023105963.0000000000000000
},
{
"_id" : ObjectId("56214d8432001bae07a6e6b9"),
"sender_id" : 2,
"receiver_id" : 8,
"content" : "qwqwqwq",
"state" : 1,
"timestamp" : 1445023108202.0000000000000000
},
{
"_id" : ObjectId("56214db032001bae07a6e6ba"),
"sender_id" : 9902,
"receiver_id" : 2,
"content" : "fsafa",
"state" : 1,
"timestamp" : 1445023152297.0000000000000000
}
I'm trying to get all unique users ids that had been messaging with user 2, along with the last content message. So the result should be:
[ { user: 8, lastContent: "qwqwqwq" }, { user: 9902, lastContent: "fsafa" } ]
By now, I have the following code:
db.getCollection('messenger').group({
keyf: function(doc) {
return { user: doc.user };
},
cond: {
$or : [
{ sender_id : 2 },
{ receiver_id : 2 }
]
},
reduce: function( curr, result ) {
result.user = (curr.sender_id == 2 ? curr.receiver_id : curr.sender_id);
result.content = curr.content;
},
initial: { } })
But I only get the last id. The result:
{
"0" : {
"user" : 9902.0000000000000000,
"content" : "fsafa"
} }
Can anyone help me with this?

Answer 1

You need to use the .aggregate() method. You need to reduce the size of documents in the pipeline using the $match operator which filter out all documents where the receiver_id is not equal to 2. After that you need to $sort your document by timestamp in descending order this will help us get the content of last message sent. Now comes the $group stage where you group your documents and use the $addToSet operator which returns array of distinct sender_id and distinct receiver_id and the $last operator to get the last message content. Now to get the user_ids we need union of distinct sender_id and receiver_id which we can get after $projection using the $setUnion operator.
db.messenger.aggregate([
{ "$match": {
"$or": [
{ "sender_id": 2 },
{ "receiver_id": 2 }
]
}},
{ "$sort": { "timestamp": 1 } },
{ "$group": {
"_id": null,
"receiver_id": {
"$addToSet": { "$receiver_id" }
},
"sender_id": {
"$addToSet": { "$sender_id" }
},
"lastContent": { "$last": "$content" }
}},
{ "$project": {
"_id": 0,
"lastContent": 1,
"user_ids": {
"$setUnion": [
"$sender_id",
"$receiver_id"
]
}
}}
])
Which returns:
{ "lastContent" : "fsafa", "user_ids" : [ 9902, 2, 8 ] }
Now if what you want is distinct user alongside their last content message with user 2 then here it is:
db.messenger.aggregate([
{ "$match": {
"$or": [
{ "sender_id": 2 },
{ "receiver_id": 2 }
]
}},
{ "$sort": { "timestamp": 1 } },
{ "$group": {
"_id": {
"sender": "$sender_id",
"receiver": "$receiver_id"
},
"lastContent": {
"$last": "$content"
},
"timestamp": { "$last": "$timestamp" },
"sender": { "$addToSet": "$sender_id" },
"receiver": { "$addToSet": "$receiver_id" }
}},
{ "$project": {
"_id": 0,
"user": {
"$setDifference": [
{ "$setUnion": [ "$sender", "$receiver" ] },
[ 2 ]
]
},
"lastContent": 1,
"timestamp": 1
}},
{ "$unwind": "$user" },
{ "$sort": { "timestamp": 1 } },
{ "$group": {
"_id": "$user",
"lastContent": { "$last": "$lastContent" }
} }
])
Which yields:
{ "_id" : 9902, "lastContent" : "fsafa" }
{ "_id" : 8, "lastContent" : "qwqwqwq" }