Modify mongoose response data - node.js

Question

I have a an API call that returns all messages a user has recieved as JSON.
The model data looks something like this:
{
sender: ObjectId,
reciever: ObjectId,
message: String
}
What is the proper way to modify the JSON the API responds with?
I want to end up with data grouped like so:
{
<senderid>:[ all of the messages from this sender],
<other_sender>:[ all of the messages from this sender]
}
Do I have to manually do this in javascript, or is there a faster way to do this taking advantage of mongoose?

Answer 1

Using the aggregation framework will be ideal for this task. You could run the following aggregation pipeline that makes use of the $group operator step to group the data to process them. The group pipeline operator is similar to the SQL's GROUP BY clause. In SQL, you can't use GROUP BY unless you use any of the aggregation functions. The same way, you have to use an aggregation function in MongoDB as well. In this instance, use the $push accumulator operator to create the array of messages.
Since Model.aggregate() returns plain objects you would then transform the resulting array to the desired hash key using lodash library's _.indexBy() method:
var pipeline = [
{
"$group": {
"_id": "$sender",
"messages": { "$push": "$message" }
}
}
];
Model.aggregate(pipeline,
function(err, res) {
if (err) return handleError(err);
var hashmap = _.chain(res)
.indexBy('_id')
.mapValues('messages')
.value();
console.log(JSON.stringify(hashmap, undefined, 4));
}
);
// Or use the aggregation pipeline builder.
Model.aggregate()
.group({ "_id": "$sender", "messages": { "$push": "$message" } })
.exec(function (err, res) {
if (err) return handleError(err);
var hashmap = _.chain(res)
.indexBy('_id')
.mapValues('messages')
.value();
console.log(JSON.stringify(hashmap, undefined, 4));
});
Check the demo below.
var data = [
{ _id: 'user1', messages: ['msg1', 'msg2'] },
{ _id: 'user2', messages: ['msg3', 'msg1'] },
{ _id: 'user3', messages: ['msg6', 'msg3'] },
{ _id: 'user4', messages: ['msg4', 'msg8'] }
];
var hashmap = _.chain(data)
.indexBy('_id')
.mapValues('messages')
.tap(log)
.value();
function log(value) {
pre.innerHTML += JSON.stringify(value, null, 4) + "\n"
}
<script src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.min.js"></script>
<pre id="pre"></pre>