Mongoose / Mongodb mapReduce or aggregate to group by single field

Question

Mongoose / Mongodb mapReduce or aggregate to group by single field

I have some documents of the form below in my conversations collection which hold a chat between multiple people. threadId holds id of one of the users in conversation between 2.

Conversation between 51c0c35b7daabc3410000001 and 51c0c35b7daabc3410000002

{
    "from" : ObjectId("51c0c35b7daabc3410000001"), 
    "to" : ObjectId("51c0c35b7daabc3410000002"),
    "threadId" : ObjectId("51c0c35b7daabc3410000002"),
    "message" : "Hi there!"
}

{
    "from" : ObjectId("51c0c35b7daabc3410000002"),
    "to" : ObjectId("51c0c35b7daabc3410000001"),
    "threadId" : ObjectId("51c0c35b7daabc3410000002"),
    "message" : "How are you?"
}

{
    "from" : ObjectId("51c0c35b7daabc3410000001"), 
    "to" : ObjectId("51c0c35b7daabc3410000002"),
    "threadId" : ObjectId("51c0c35b7daabc3410000002"),
    "message" : "I am good. How are you?"
}

Conversation between 51c0c35b7daabc3410000001 and 51c0c69989a44a6812000003

{
    "from" : ObjectId("51c0c35b7daabc3410000001"), 
    "to" : ObjectId("51c0c69989a44a6812000003"),
    "threadId" : ObjectId("51c0c69989a44a6812000003"),
    "message" : "Hello!"
}

{
    "from" : ObjectId("51c0c69989a44a6812000003"), 
    "to" : ObjectId("51c0c35b7daabc3410000001"),
    "threadId" : ObjectId("51c0c69989a44a6812000003"),
    "message" : "Hey.."
}

I want to group based on threadId and the output should be of the form

{
    "from" : ObjectId("51c0c35b7daabc3410000001"), 
    "to" : ObjectId("51c0c35b7daabc3410000002"),
    "threadId" : ObjectId("51c0c35b7daabc3410000002"),
    "message" : "I am good. How are you?"
}

{
    "from" : ObjectId("51c0c69989a44a6812000003"), 
    "to" : ObjectId("51c0c35b7daabc3410000001"),
    "threadId" : ObjectId("51c0c69989a44a6812000003"),
    "message" : "Hey.."
}

So I want to find all the documents with a distinct threadId (or group based only on threadId). The output should also include all the other fields in the document.

mapReduce, aggregate/group, distinct or a combination of these ? How do I go about it ?

EDIT

To put it in a SQL statement, I want something as simple as

SELECT * FROM conversations GROUP BY threadId;

node.js
mongodb
mongoose

Answer 1

I ended up doing this

// Aggregate pipeline
Conversation.aggregate(
    { $match: {
        listing: new ObjectId(listingId)
      }
    },
    { $group: {
        _id: '$threadId', 
        message: { $last: "$message" },
        to: { $last: "$to" },
        from: { $last: "$from" }
      }
    },
    { $project : {
        _id : 0,
        threadId : "$_id",
        message: "$message",
        to: "$to",
        from: "$from"
      }
    },
    function(err, threads) {
        console.log(err);
        console.log(threads);
    }
);

It seems to work fine. Let me know if there is any simpler way or if this snippet can be optimized. Hope this helps someone.