MongoDb: aggregation $lookup - mongodb

Question

Given following collections.
I have to get the title field and combine to identifier
CREDENTIAL:
{
_id: ..
title: ..
}
USER_CREDENTIAL:
{
_id: ..
credential_id: .. (from credential collection)
created_at: ..
identifier: {
first_name: ..
middle_name: ..
last_name: ..
}
}
The response should be:
{
user_credential_id:
member: {
first_name:
middle_name:
last_name:
title:
created_at:
}
}

Answer 1

$lookup - Join user and user_credential_id with a pipeline to match the condition ($match) and decorate the document ($project).
$unwind - Deconstruct member array to multiple documents.
$project - Decorate the output document.
db.user.aggregate([
{
"$lookup": {
"from": "user_credential",
let: {
"user_credential_id": "$_id",
"title": "$title"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$credential_id",
"$$user_credential_id"
]
}
}
},
{
$project: {
first_name: "$identifier.first_name",
middle_name: "$identifier.middle_name",
last_name: "$identifier.last_name",
title: "$$title",
created_at: "$created_at",
}
}
],
"as": "member"
}
},
{
$unwind: "$member"
},
{
$project: {
_id: 0,
user_credential_id: "$_id",
member: 1
}
}
])
Sample Mongo Playground

Answer 2

You can try this query
db.user.aggregate([
{
$lookup: {
from: "credentials",
localField: "credential_id",
foreignField: "_id",
as: "user_credential"
}
},
{
$unwind: "$user_credential"
},
{
$project: {
_id: 0,
user_credential_id: "$credential_id",
member: {
first_name: "$identifier.first_name",
middle_name: "$identifier.middle_name",
last_name: "$identifier.last_name",
title: "$user_credential.title",
created_at: "$created_at",
}
}
}
])
You can check it out here