MongoDB Grouping Query - mongodb

Question

I'm working on a Meteor application and I have data for a weekly timetable of the format -
events:
event:
day: "Monday"
time: "9am"
location: "A"
event:
day: "Monday"
time: "10am"
location: "B"
There are numerous entries for each day. Can I run a query that will return an object of the format -
day: "Monday"
events:
event:
time: "9am"
location: "A"
event:
time: "10am"
location: "B"
I could store the object in the second format but prefer the first for ease of deleting and updating individual events.
I also want to return them ordered by day of week if there's a nice way to do that.

Answer 1

Several options:
You can use an aggregation command but be warned, you will loose reactivity: it means that except when you reload your template, you will not get external updates. You would also need to use a package to add the aggregation command to Meteor in order to achieve that.
My personal favorite: you don't need to aggregate (and loose reactivity) to achieve your data transformation. You can use a simple Collection.find() query and extend/reduce/modify it using a clever mix of cursor.Observe() and conditional modifications. Have a look at this answer, it did the trick for me (I needed a sum with black listing of some fields, but you can easily adapt it to your group/sorting case) : https://stackoverflow.com/a/30813050/3793161. Comment if you need more details on this
If you plan to have several servers, be warned that each server will have to observe so it may lead to an unnecessary load. So my third solution is either use collection hooks or methods to update an additional field containing every event for each day/user (whatever you need).See #David Weldon answer about that here: https://stackoverflow.com/a/31190896/3793161. In you case, it would probably mean to re-think your database structure to fit your needs (i.e. adding more fields so you ca update them on insert.
EDIT Here are some thoughts on your comment:
If you stick to what you described in the question, you would need seven documents, one per day, with an events field where you put all the events. My second solution is good if you need to rework a collection structure before sending it. However, in your case, you just need an object week with 7 sub-objects matching the days of the week.
I advise you to possible approaches:
use the aggregation in a method, as described by #chridam. Be warned that you will not be able to directly get a sorted array, as stated in mongo $group documentation
$group does not order its output documents
So you need to sort them (by day and by hour within each day) using, for example _.sortBy() before you return your method result. By the way, if you want to know what is going on in your method call, clientside, here is how you should write the call:
Meteor.call("getGroupedDailyEvents", userId, function(error, result){
if(error){
console.log("error", error);
}
if(result){
//do whatever you need
}
});
Make the data sorting client-side. You are looking for an overkill solution because, afaik, you don't need to filter any data to keep it from the user, and you are going to send the data anyway (just with another structure). This is much easier to make a simple helper in your template like this:
Template.displaySchedule.helpers({
"monday_events": function() {
return _.sortBy (events.find({day:"Monday"}).fetch(), "time")
},
//add other days
);
It assumes the format of your time field is sortable this way. If not, you just need to create a function to sort them accordingly to their formats or change the original format into something better suited.
The rest (HTML) would just be to iterate on Monday events using a {{#each monday_events}}

Answer 2

To achieve the desired result, use the aggregation framework where the $group pipeline operator groups all the input documents and apply the accumulator expression $push to the group to get the events array.
Your pipeline would look like this:
var Events = new Mongo.Collection('events');
var pipeline = [
{
"$group": {
"_id": "$day",
"events": {
"$push": {
"time": "$time"
"location": "$location"
}
}
}
},
{
"$project": {
"_id": 0, "day": "$_id", "events": 1
}
}
];
var result = Events.aggregate(pipeline);
console.log(result)
You can add the meteorhacks:aggregate package to implement the aggregation in Meteor:
Add to your app with
meteor add meteorhacks:aggregate
Since this package exposes .aggregate method on Mongo.Collection instances, you can define a method that gets the aggregated result array. For example
if (Meteor.isServer) {
var Events = new Mongo.Collection('events');
Meteor.methods({
getGroupedDailyEvents: function () {
var pipeline = [
{
"$group": {
"_id": "$day",
"events": {
"$push": {
"time": "$time"
"location": "$location"
}
}
}
},
{
"$project": {
"_id": 0, "day": "$_id", "events": 1
}
}
];
var result = Events.aggregate(pipeline);
return result;
}
});
}
if (Meteor.isClient) {
Meteor.call('getGroupedDailyEvents', logResult);
function logResult(err, res) {
console.log("Result: ", res)
}
}