Mongodb aggregate lookup three collections - mongodb

Question

Learning MongoDB for the past two days and I am trying to aggregate three collections but unable to achieve it
Below are the three collection maintaining in the database
t_credentials
{
"_id" : "619ca68b624c41e408348406",
"title" : "Company ID"
}
t_groups
{
"_id" : "61a253da88ca12a37218898d",
"group_name" : "Gold"
}
t_user_credentials
{
"_id" : "619ca88a624c41e408348424",
"credential_id" : "619ca68b624c41e408348406",
"group_id" : "61a253da88ca12a37218898d",
"identifiers" : {
"first_name" : "Lee",
"middle_name" : "Min",
"last_name" : "Ho"
},
"created_at" : "2021-12-01T17:20:49.000Z"
}
Here I am trying to achieve the output in the below format:
Expected Output
[{
"_id" : "619ca88a624c41e408348424",
"first_name" : ,
"middle_name" : ,
"last_name" : ,
"credential" : {
"_id:" : "619ca68b624c41e408348406",
"title" : "Company ID"
},
"group" : {
"_id" : "61a253da88ca12a37218898d",
"group_name" : "Gold"
},
"created_at" : "2021-12-01T17:20:49.000Z"
}]
But, I am getting the fields only from t_user_credentials but not able to get like in the above format
Query
db.t_user_credentials.aggregate([
{
$lookup: {
from: "t_credentials",
localField: "_id",
foreignField: "credential_id",
as: "credentials"
}
},
{
$unwind: {
path:'$credentials',
preserveNullAndEmptyArrays: true
}
},
{
$lookup: {
from: "t_groups",
localField: "_id",
foreignField: "group_id",
as: "groups"
}
},
{
$unwind: {
path: '$groups',
preserveNullAndEmptyArrays: true
}
},
{
$project: {
last_name: "$identifiers.last_name",
first_name: "$identifiers.first_name",
middle_name: "$identifiers.middle_name",
"credentials.title": 1,
created_at: 1,
group_id: 1
}
}
])
Can any one help me to solve this issue, it will be very helpful for me.

Answer 1

This query uses $replaceWith to merge the identifiers sub-document into the $$ROOT document. We also use $unset to remove fields we are no longer interested in. Before all of that we make sure to unwind our credential and group fields.
You can check out a live demo of this query here
Consider the following:
Database
db={
"t_credentials": [
{
"_id": "619ca68b624c41e408348406",
"title": "Company ID"
}
],
"t_groups": [
{
"_id": "61a253da88ca12a37218898d",
"group_name": "Gold"
}
],
"t_user_credentials": [
{
"_id": "619ca88a624c41e408348424",
"credential_id": "619ca68b624c41e408348406",
"group_id": "61a253da88ca12a37218898d",
"identifiers": {
"first_name": "Lee",
"middle_name": "Min",
"last_name": "Ho"
},
"created_at": "2021-12-01T17:20:49.000Z"
}
]
}
Query
db.t_user_credentials.aggregate([
{
"$lookup": {
"from": "t_credentials",
"localField": "credential_id",
"foreignField": "_id",
"as": "credential"
}
},
{
"$lookup": {
"from": "t_groups",
"localField": "group_id",
"foreignField": "_id",
"as": "group"
}
},
{
$unwind: "$group",
},
{
$unwind: "$credential"
},
{
$replaceWith: {
$mergeObjects: [
"$$ROOT",
"$identifiers"
]
}
},
{
$unset: [
"group_id",
"credential_id",
"identifiers"
]
}
])
Result
[
{
"_id": "619ca88a624c41e408348424",
"created_at": "2021-12-01T17:20:49.000Z",
"credential": {
"_id": "619ca68b624c41e408348406",
"title": "Company ID"
},
"first_name": "Lee",
"group": {
"_id": "61a253da88ca12a37218898d",
"group_name": "Gold"
},
"last_name": "Ho",
"middle_name": "Min"
}
]