How to get friend's leaderboard in MongoDB - node.js

Question

This is my Friends Collection
[
{
"_id": "59e4fbcac23f38cdfa6963a8",
"friend_id": "59e48f0af8c277d7a8886ed7",
"user_id": "59e1d36ad17ad5ad3d0453f7",
"__v": 0,
"created_at": "2017-10-16T18:34:50.875Z"
},
{
"_id": "59e5065f705a90cfa218c9e5",
"friend_id": "59e48f0af8c277d7a8886edd",
"user_id": "59e1d36ad17ad5ad3d0453f7",
"__v": 0,
"created_at": "2017-10-16T19:19:59.483Z"
}
]
This is my Scores collection:
[
{
"_id": "59e48f0af8c277d7a8886ed8",
"score": 19,
"user_id": "59e48f0af8c277d7a8886ed7",
"created_at": "2017-10-13T09:02:10.010Z"
},
{
"_id": "59e48f0af8c277d7a8886ed9",
"score": 24,
"user_id": "59e48f0af8c277d7a8886ed7",
"created_at": "2017-10-11T00:56:10.010Z"
},
{
"_id": "59e48f0af8c277d7a8886eda",
"score": 52,
"user_id": "59e48f0af8c277d7a8886ed7",
"created_at": "2017-10-24T09:16:10.010Z"
},
]
This is my Users collection.
[
{
"_id": "59e48f0af8c277d7a8886ed7",
"name": "testuser_0",
"thumbnail": "path_0"
},
{
"_id": "59e48f0af8c277d7a8886edd",
"name": "testuser_1",
"thumbnail": "path_1"
},
{
"_id": "59e48f0af8c277d7a8886ee3",
"name": "testuser_2",
"thumbnail": "path_2"
},
{
"_id": "59e48f0af8c277d7a8886ee9",
"name": "testuser_3",
"thumbnail": "path_3"
},
]
And finally i need list of friends sorted in highscore order for a particular time period (say last 24 hours) with something like this...
[
{
"friend_id": "59e48f0af8c277d7a8886ed7",
"friend_name":"test_user_2"
"thumbnail":"image_path",
"highscore":15
},
"friend_id": "59e48f0af8c277d7a8886edd",
"friend_name":"test_user_3"
"thumbnail":"image_path",
"highscore":10
}
]
What's the best way to achieve this? I have tried aggregation pipeline but getting quite confused with working with 3 collections.

Answer 1

Following your answers, an array size of 500 entries in a document may not be a bad idea to store the friends as you would only store "friends id" and "created" in each entry. It saves having a collection.
You would not have too much performances issues if you project the data in your query by selecting only the fields you want.
https://docs.mongodb.com/v3.2/tutorial/project-fields-from-query-results/#return-specified-fields-only
For the score that increase of 30 per day; it depends what type of query you do.
It would take a while to reach the 2MB limit per the document by adding 30 scores per day.
regarding joining the different collections there is a stack overflow question about it:
How do I perform the SQL Join equivalent in MongoDB?
or
https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/
You will need to use the aggregation framework from mongoDB to use if; not just a find command.