Add additional properties to the output json - mongodb

Question

There is a collection of User
{
"_id" : ObjectId("5a0d45ca8af3a91847b7cf95"),
"updatedAt" : ISODate("2017-11-16T09:34:14.651Z"),
"createdAt" : ISODate("2017-11-16T08:01:14.119Z"),
"name" : "John",
"email" : "test1#gmail.com",
"groupsFavorite" : [
ObjectId("5a0d45db8af3a91847b7cf96")
],
"groups" : [
ObjectId("5a0d45db8af3a91847b7cf96"),
ObjectId("5a0d45e18af3a91847b7cf97")
],
"__v" : 3
}
There is a collection of Groups
/* 1 */
{
"_id" : ObjectId("5a0d45db8af3a91847b7cf96"),
"updatedAt" : ISODate("2017-11-16T08:01:31.815Z"),
"createdAt" : ISODate("2017-11-16T08:01:31.815Z"),
"userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
"title" : "New title",
"slug" : "new-title-1",
"description" : "Lorem",
"__v" : 0
}
/* 2 */
{
"_id" : ObjectId("5a0d45e18af3a91847b7cf97"),
"updatedAt" : ISODate("2017-11-16T08:01:37.005Z"),
"createdAt" : ISODate("2017-11-16T08:01:37.005Z"),
"userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
"title" : "New title",
"slug" : "new-title-2",
"description" : "Lorem",
"__v" : 0
}
/* 3 */
{
"_id" : ObjectId("5a0d5cb0cd59342da943d55a"),
"updatedAt" : ISODate("2017-11-16T09:38:56.912Z"),
"createdAt" : ISODate("2017-11-16T09:38:56.912Z"),
"userId" : ObjectId("5a0d5c48cd59342da943d559"),
"title" : "New title",
"slug" : "new-title-3",
"description" : "Lorem",
"__v" : 0
}
There is a method that returns groups of a particular user.
It is necessary to add these data to the new property of favorite: true or false for building a table.
For example:
{
"_id" :"5a0d45db8af3a91847b7cf96",
"updatedAt" : "2017-11-16T08:01:31.815Z",
"createdAt" : "2017-11-16T08:01:31.815Z",
"userId" : "5a0d45ca8af3a91847b7cf95",
"title" : "New title",
"slug" : "new-title-1",
"description" : "Lorem",
"favorite": "true"
},
{
"_id" : "5a0d45e18af3a91847b7cf97",
"updatedAt" : "2017-11-16T08:01:37.005Z",
"createdAt" : "2017-11-16T08:01:37.005Z",
"userId" : "5a0d45ca8af3a91847b7cf95",
"title" : "New title",
"slug" : "new-title-2",
"description" : "Lorem",
"favorite": "false"
},
{
"_id" : "5a0d5cb0cd59342da943d55a",
"updatedAt" : "2017-11-16T09:38:56.912Z",
"createdAt" : "2017-11-16T09:38:56.912Z",
"userId" : "5a0d5c48cd59342da943d559",
"title" : "New title",
"slug" : "new-title-3",
"description" : "Lorem",
"favorite": "false"
}

Answer 1

You can use $lookup operator for checking the _id and userId fields in "User" collection. And also for determining it is in or not, you can use $eq operator.
db.getCollection('Groups').aggregate([
{
$lookup:
{
from: "User",
localField: "_id",
foreignField: "groupsFavorite",
as: "FavoriteByGrp"
}
}
,{
$lookup:
{
from: "User",
localField: "userId",
foreignField: "_id",
as: "FavoriteByUsr"
}
}
,{
"$project":
{
_id:1,
updatedAt:1,
createdAt:1,
userId:1,
title:1,
slug:1,
description:1,
favorite:
{
"$cond":
{
if: { "$eq": [ "$FavoriteByGrp._id", "$FavoriteByUsr._id" ] },
then: "true",
else: "false"
}
}
}
}
])
Result:
/* 1 */
{
"_id" : ObjectId("5a0d45db8af3a91847b7cf96"),
"updatedAt" : ISODate("2017-11-16T08:01:31.815Z"),
"createdAt" : ISODate("2017-11-16T08:01:31.815Z"),
"userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
"title" : "New title",
"slug" : "new-title-1",
"description" : "Lorem",
"favorite" : "true"
}
/* 2 */
{
"_id" : ObjectId("5a0d45e18af3a91847b7cf97"),
"updatedAt" : ISODate("2017-11-16T08:01:37.005Z"),
"createdAt" : ISODate("2017-11-16T08:01:37.005Z"),
"userId" : ObjectId("5a0d45ca8af3a91847b7cf95"),
"title" : "New title",
"slug" : "new-title-2",
"description" : "Lorem",
"favorite" : "false"
}
/* 3 */
{
"_id" : ObjectId("5a0d5cb0cd59342da943d55a"),
"updatedAt" : ISODate("2017-11-16T09:38:56.912Z"),
"createdAt" : ISODate("2017-11-16T09:38:56.912Z"),
"userId" : ObjectId("5a0d5c48cd59342da943d559"),
"title" : "New title",
"slug" : "new-title-3",
"description" : "Lorem",
"favorite" : "false"
}