Parent child reversal in the result - mongodb

Question

I am using MongoDB 3.4.
Have 2 collection as follows.
Collection 1:- type
{
"_id": {
"$numberLong": "1234"
},
"name" : "board1"
"type" : "electronic"
},
{
"_id": {
"$numberLong": "1235"
},
"name" : "board2",
"type" : "electronic"
}
Collection 2:- products
{
"_id": {
"$numberLong": "9876"
},
"types" : [
"1234",
"1235",
"1238"
]
},
{
"_id": {
"$numberLong": "9875"
},
"types" : [
"1234",
"1238"
]
}
Type collection will have multiple types and each product in products collection will have multiple types.
There can be multiple document with different ids for the same type in type collection. And, product collection might have types array with different Ids of same type or different type.
I would like to get all the ids of type electronic and find the products which has id in the types array for each product.
I want result like the below one.
{
"_id": {
"$numberLong": "1234"
},
"name" : "board1",
"products" : [
"9876",
"9875"
]
},
{
"_id": {
"$numberLong": "1235"
},
"name" : "board2"
"products" : [
"9876",
"9875"
]
}
Currently, I am making so many calls, like for each type id, get all products.
Is there any other simple way with single query using $lookup or any other mechanism?

Answer 1

You can try below aggregation in mongodb 3.6 and above
db.types.aggregate([
{ "$match": { "type" : "electronic" }},
{ "$lookup": {
"from": "testCollection2",
"let": { "typeId": "$_id" },
"pipeline": [
{ "$match": { "$expr": { "$in": ["$$typeId", "$types"] }}}
],
"as": "products"
}},
{ "$addFields": {
"products": "$products._id"
}}
])
You can try bbelow aggregation in mongodb 3.4
db.types.aggregate([
{ "$match": { "type" : "electronic" }},
{ "$lookup": {
"from": "testCollection2",
"localField": "_id",
"foreignField": "types",
"as": "products"
}},
{ "$addFields": {
"products": "$products._id"
}}
])

Answer 2

In MongoDB 3.4 you can use $lookup and then $addFields to get _id from products:
db.types.aggregate([
{
"$match": { "type" : "electronic" }
},
{
$lookup: {
from: "products",
localField: "_id",
"foreignField": "types",
"as": "products"
}
},
{
$project: {
field1: 1,
field2: 1,
products: {
$map: {
input: "$products",
as: "p",
in: "$$p._id"
}
}
}
}
])