Find events associated with users on a certain date in MongoDB

Question

I have this script which works perfectly but I experienced some delays because of these 2 for loops [i][j]. Is there any way to do the same function but with a better and more effective process like foreach or other?

User.find({}).lean(true).exec((err, users) => {
        let getTEvent = [];
        //nested loops() //callbacks
   
        for (let i =0 ; i < users.length; i++) { 
          if(users[i].events && users[i].events.length) {
const dt = datetime.create();
dt.offsetInDays(0);
const formatted = dt.format('d/m/Y');
// console.log(formatted)
for (let j = 0; j < users[i].events.length; j++) {
    if(users[i].events[j].eventDate === formatted) {
      getTEvent.push({events: users[i].events[j]});
      }  
    }
  }
}


   return res.json(getTEvent)
    }); 

The main role of this code is:

1. find the data in Mongodb: find all users with or without events

2. loop through data and select events made by any users

3. push the results in an array which is getEventin order to be useful later for the client side

Details:

The User is the model: const User = require('../models/users.model'); and events is [] array.

This is Mongodb structure of that models:

Answer 1

I think the main cause of your delays is not a nested for but rather the fact that you extract all data from your MongoDB collection into memory. What you can do is:

1. calculate formatted date once

2. query just those users that contain given event date instead of populating all users in memory. Here's the guide.

This will look roughly like the following:

const dt = datetime.create();
dt.offsetInDays(0);
const formatted = dt.format('d/m/Y');

User.find({
        "events": {
            eventDate: formatted
        }       
    }).lean(true).exec((err, users) => {
        let getTEvent = [];
        //nested loops() //callbacks

        for (let i = 0 ; i < users.length; i++) { 
          if(users[i].events && users[i].events.length) {           
            for (let j = 0; j < users[i].events.length; j++) {
                if(users[i].events[j].eventDate === formatted) {
                  getTEvent.push({events: users[i].events[j]});
                }  
            }
          }
        }


   return res.json(getTEvent)
}); 

Answer 2

I don't fully understand what you are trying to achieve with this code, and you haven't really described it very well. I'm reasonably sure, though, that all this code should be completely eliminated in favour of a MongoDB query, so that the processing is done by the database without shipping the full dataset to the client.

Answer 3

I was planning to suggest filtering the users in the query, as Bohdan suggested. Below are more suggestions.

Variable declarations

Another suggestion is to default to using const for all variables including arrays. This helps avoid accidental re-assignment and other bugs. If you determine that you need to re-assign a variable then switch it to let.

The variable getTEvent can be declared with const since it is never re-assigned. If you had to remove all elements the length property could be set to 0.

looping apporach

Also you can use for...of loops instead of for loops when the index is not used for anything other than selecting a current index. You could also consider a functional approach with array.filter() - this would avoid the need to push filtered items manually.