Mongoose join two collections and get referenced data in two properties - node.js

Question

I'm doing a simple follow friend functionality.
Please see my codes below:
Following schema:
{
"userId": { type: String },
"followers": [{ "followerId": type: String }],
"followings": [{ "followingId": type: String }],
}
User schema:
{
"fullName": { type: String }
}
Note: user 8 has 1 follower and 2 followings.
Now, my expected output should be like this:
"userId": 8,
"followers": [
{
"followerId": 4,
"fullName": "Rose Marriott",
},
{
"followerId": 5,
"fullName": "James Naismith",
}
],
"followings": [
{
"followingId": 1,
"fullName": "Russell Oakham",
},
{
"followingId": 5,
"fullName": "James Naismith",
}
]
This is what I tried so far:
db.followings.aggregate([
{ $unwind: "$followers" },
{
$lookup: {
from: "users",
localField: "followers.followerId",
foreignField: "_id",
as: "users"
}
},
{
$addFields:
{
users: { $arrayElemAt: ["$users", 0] },
},
},
{ $unwind: "$followings" },
{
$lookup: {
from: "users",
localField: "followings.followingId",
foreignField: "_id",
as: "users2"
}
},
{
$addFields:
{
users2: { $arrayElemAt: ["$users2", 0] },
},
},
{ $match: {"userId": mongoose.Types.ObjectId(userId) } },
{
$group: {
_id: "$_id",
userId: { $first: "$userId" },
followers: {
$push: {
followerId: "$followers.followerId",
fullName: "$users.fullName",
}
},
followings: {
$push: {
followingId: "$followings.followingId",
fullName: "$users2.fullName",
}
}
}
}
]);
But I'm getting 2 followers and 2 followings. I wonder what's causing this issue. Appreciate any help. Thanks!

Answer 1

You can try,
$addFields to make a unique array called userIds form both arrays followers and followings, $setUnion to get unique ids,
$lookup with users collection
$project to show fields,
followers get fullName, $map to iterate loop of followers and get the name of followerId from users array using $reduce and $cond
followings get fullName, $map to iterate loop of followings and get the name of followingId from users array using $reduce and $cond
db.followings.aggregate([
{
$addFields: {
userIds: {
$setUnion: [
{
$map: {
input: "$followers",
in: "$$this.followerId"
}
},
{
$map: {
input: "$followings",
in: "$$this.followingId"
}
}
]
}
}
},
{
$lookup: {
from: "users",
localField: "userIds",
foreignField: "_id",
as: "users"
}
},
{
$project: {
userId: 1,
followers: {
$map: {
input: "$followers",
as: "f",
in: {
$mergeObjects: [
"$$f",
{
fullName: {
$reduce: {
input: "$users",
initialValue: "",
in: {
$cond: [
{ $eq: ["$$this._id", "$$f.followerId"] },
"$$this.fullName",
"$$value"
]
}
}
}
}
]
}
}
},
followings: {
$map: {
input: "$followings",
as: "f",
in: {
$mergeObjects: [
"$$f",
{
fullName: {
$reduce: {
input: "$users",
initialValue: "",
in: {
$cond: [
{ $eq: ["$$this._id", "$$f.followingId"] },
"$$this.fullName",
"$$value"
]
}
}
}
}
]
}
}
}
}
}
])
Playground

Answer 2

when working with relations on Mongoose you should create the relationship based on a unique id, and then reference the document. In your case, it would be:
followingSchema = new mongoose.Schema({
{
"followers": [{type: mongoose.Schema.types.ObjectId, ref="User"}],
"followings": [{type: mongoose.Schema.types.ObjectId, ref="User"}],
}
})
userSchema = new mongoose.Schema({
fullname: String,
})
be aware that the userId will be created automatically by Mongoose with a field called _id. so, the end result of creating a new following documents would be:
{
_id: "klajdsfñalkdjf" //random characters created by mongoose,
followers: ["adfadf134234", "adfadte345"] //same as before, these are Ids of the users randomly generated by mongoose
followers: ["adfadf134234", "adfadte345"]
}
{
_id: adfadf134234,
fullName: "alex",
}
now, because there is no use for us to have a random number as information in the fields of following and followers in the following object, we can now use the method .populate() that can be used over the document itself to transform those Ids into actual information. You can see more about it here: mongoose documentation
our final result would be something like this:
{
_id: "añfakdlsfja",
followers : [{_id: "adlfadsfj", fullName: "alex"}],
following : [{_id: "adfadfad" , fullName: "other"}, {_id: "dagadga", fullName: "another"}]
}