MongoDB Lookup in array - mongodb

Question

Good Morning.
I'm trying to query mongoDB by joining two collections. But I'm not able to do that when the primary identifier is inside an array in the secondary collection.
See my code and my collection
Pipe Colletion
{ _id: 1, deal_id: 25698}
{ _id: 2, deal_id: 45879}
{ _id: 3, deal_id: 54142}
Leads Colletion
{ _id: 1, name:"Teste A", deals_id:[25698,45879]}
{ _id: 2, name:"Teste B", deals_id:[54142]}
Desired result when searching for deal_id from the Pipe collection:
Results
{ _id: 1, deal_id: 25698, name:"Teste A"}
{ _id: 2, deal_id: 45879, name:"Teste A"}
{ _id: 3, deal_id: 54142, name:"Teste B"}
My Code:
db.pipedrive.aggregate([
{
$lookup: {
from: "leads",
'let': {
deal_id: '$deal_id'
},
pipeline: [
{
$match: {
$expr: {
$and: [
{$eq: ["$$deal_id", "$deals_id"]}
]
}
}
}
],
as: "Results"
}
}
]);
As I am currently doing, the analytics line is always returning me empty.
Can you help me?

Answer 1

With your original approach, exchanging $eq with $in should work, and since you only have one condition $and is not necessary. Note that $deals_id should exists in foreign document and must be an array, as $in requires the second parameter to be an array. So in case it doesn't exist we will have to wrap it with $ifNull
db.pipedrive.aggregate([{
$lookup: {
from: "leads",
let: {
deal_id: '$deal_id'
},
pipeline: [
{
$match: {
$expr: { $eq: ["$$deal_id", { $ifNull: ["$deals_id", []] ] }
}
}
],
as: "Results"
}
}
// optional other stages to output as desired shape
]);
A Better alternative would be just to use regular $lookup, since mongodb is able to compare the with the elements of the foreign field array. Just like with find query. This will also account for missing fields or non array fields.
db.pipedrive.aggregate([{
$lookup: {
from: "leads",
localField: "deal_id",
foreignField: "deals_id",
as: "Results"
}
}
// optional other stages to output as desired shape
{
$unwind: "$Results"
},
{
$project: {
deal_id: true,
name: "$Results.name"
}
}
]);
Mongo playground

Answer 2

You can use this query to get desired results. $lookup will take care of array as well.
db.pipe.aggregate([
{
$lookup: {
from: "leads",
localField: "deal_id",
foreignField: "deals_id",
as: "results"
}
},
{
$unwind: "$results"
},
{
$project: {
_id: 1,
deal_id: 1,
name: "$results.name"
}
}
])
Here is the link to check results,
https://mongoplayground.net/p/mhF6zHD9d26