Join a collection to an existing aggregate query mongodb - node.js

Question

I have the following collection structures in my mongodb database.
Orders
[
{
"_id":"order_abcd",
"name":"Order 1"
},
{
"_id":"order_defg",
"name":"Order 2"
}
]
Session
{
"_id":"session_abcd"
"orders": [ ObjectId("order_abcd"), ObjectId("order_defg") ]
}
Transactions
{
"_id":"transaction_abcd"
"id_session" : ObjectId("session_abcd")
}
What I am trying to achieve is a dataset that looks similar to this
[
{
"_id":"order_abcd",
"name":"Order 1",
"transaction":"transaction_abcd"
},
{
"_id":"order_defg",
"name":"Order 2",
"transaction":"transaction_abcd"
}
]
I do not have any input data other than a start date and an end date which will be used to filter orders, the query is mostly for reporting purposes so in effect I am trying to generate a query to fetch all orders between a given time period and attach the transaction id for each order to it.

Answer 1

We can use couple of $lookup (analogous to join with 2 tables in SQL) with $unwind at each stage to finally $project the key-value pair that is desired.
db.Session.aggregate([
{
$lookup: {
from: "Transactions",
localField: "_id",
foreignField: "id_session",
as: "transaction_info"
}
},
{ $unwind: "$transaction_info" },
{
$lookup: {
from: "Orders",
localField: "orders",
foreignField: "_id",
as: "order_info"
}
},
{ $unwind: "$order_info" },
{$project:{
_id:"$order_info._id",
name:"$order_info.name",
transaction:"$transaction_info._id"
}}
]).pretty();
Which gives an output:
{
"_id" : "order_abcd",
"name" : "Order 1",
"transaction" : "transaction_abcd"
},
{
"_id" : "order_defg",
"name" : "Order 2",
"transaction" : "transaction_abcd"
}
The unwind stages are used to explode the lookup and then cherry pick fields at final project stage.
++UPDATE++
Another option that probably can help reduce the 2nd stage lookup records since $match on dates of Orders can be applied to pass on filtered docs for next stage.
db.Session.aggregate([
{
$lookup: {
from: "Orders",
localField: "orders",
foreignField: "_id",
as: "order_info"
}
},
{ $unwind: "$order_info" },
{
$match: {} //filter on "order_info.property" (i:e; date,name,id)
},
{
$lookup: {
from: "Transactions",
localField: "_id",
foreignField: "id_session",
as: "transaction_info"
}
},
{ $unwind: "$transaction_info" },
{
$project: {
_id: "$order_info._id",
name: "$order_info.name",
transaction: "$transaction_info._id"
}
}
]).pretty();