How to perform nested lookup in mongo query? - mongodb

Question

This is my aggregation query
db.user.aggregate([
{ $addFields: { user_id: { $toString: "$_id" } } },
{
$lookup: {
from: "walletData",
let: { id: "$user_id" },
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: ["$userId", "$$id"]
},
{
$gt: ["$lastBalance", 0]
}
]
}
}
}
],
as: "balance"
}
}
])
I get the desired output from this result but need to join one more collection in this query. How can i achieve that?
For example consider these collections:
user : {
"_id": ObjectId("xyz")
}
walletData:{
"userId": "xyz",
"lastBalance": 5
}
AnotherWalletdata:{
"userId": "xyz",
"lastBalance": 6
}
I got the result after joining first two tables how do i join the third table only if the balance of the second table(walletData) is greater than zero?
Expected Output :
{"id":"xyz",
"walletdataBal":5,
"AnotherWalletDataBal":6
}

Answer 1

You can join any number of collections by using only $lookup and $unwind one after another followed by Conditional Projection for whatever that's required at last. Below is the well-tested and working solution for the same :
db.user.aggregate([
{$lookup: {from: "walletData", localField: "_id", foreignField: "userId", as: "walletDataBal"}},
{$unwind: "$walletDataBal"},
{$lookup: {from: "anotherwalletData", localField: "_id", foreignField: "userId", as: "anotherWalletDataBal"}},
{$unwind: "$anotherWalletDataBal"},
{$project: {"id": "$_id", "_id": 0, walletDataBal: "$walletDataBal.lastBalance",
anotherWalletDataBal: {$cond:
{if: { $gt: [ "$walletDataBal.lastBalance", 0 ] },
then: "$anotherWalletDataBal.lastBalance",
else: "$$REMOVE" }}}
]).pretty();

Answer 2

You can add another $lookup stage to achieve the output
db.user.aggregate([
{ "$addFields": { "user_id": { "$toString": "$_id" } } },
{ "$lookup": {
"from": "walletData",
"let": { "id": "$user_id" },
"pipeline": [
{ "$match": {
"$expr": {
"$and": [
{ "$eq": ["$userId", "$$id"] },
{ "$gt": ["$lastBalance", 0] }
]
}
}}
],
"as": "balance"
}},
{ "$lookup": {
"from": "anotherWalletData",
"let": { "id": "$user_id" },
"pipeline": [
{ "$match": {
"$expr": {
"$and": [
{ "$eq": ["$userId", "$$id"] },
{ "$gt": ["$lastBalance", 0] }
]
}
}}
],
"as": "anotherWalletData"
}},
{ "$project": {
"walletdataBal": { "$arrayElemAt": ["$balance.lastBalance", 0] },
"anotherwalletdataBal": {
"$arrayElemAt": ["$anotherWalletData.lastBalance", 0]
}
}}
])

Answer 3

{ $unwind: "$balance" },
{ $lookup: {
from: "walletData",
localField: "balance.userId",
foreignField: "userId",
as:"anotherwalletData"
}}
])
I solved my query, I had to apply unwind after the above lookup.