How can I perform nested "joins" (joining 3 or more collections) in a MongoDB aggregation pipeline? - mongodb

Question

Let's say we have 3 hypothetical collections in MongoDB: customers, orders, and orderItems.
Each customer has multiple orders, and each order has multiple order items.
Here's some sample data for these 3 collections:
customers
[
{
customer_id: 1,
name: "Jim Smith",
email: "jim.smith#example.com"
},
{
customer_id: 2,
name: "Bob Jones",
email: "bob.jones#example.com"
}
]
orders
[
{
order_id: 1,
customer_id: 1
},
{
order_id: 2,
customer_id: 1
}
]
orderItems
[
{
order_item_id: 1,
name: "Foo",
price: 4.99,
order_id: 1
},
{
order_item_id: 2,
name: "Bar",
price: 17.99,
order_id: 1
},
{
order_item_id: 3,
name: "baz",
price: 24.99,
order_id: 2
}
]
Desired Result
How can I write my aggregation pipeline so that the result returned looks something like this?
[
{
customer_id: 1,
name: "Jim Smith",
email: "jim.smith#example.com"
orders: [
{
order_id: 1,
items: [
{
name: "Foo",
price: 4.99
},
{
name: "Bar",
price: 17.99
}
]
},
{
order_id: 2,
items: [
{
name: "baz",
price: 24.99
}
]
}
]
},
{
customer_id: 2,
name: "Bob Jones",
email: "bob.jones#example.com"
orders: []
}
]

Answer 1

Do nested lookup using lookup with pipeline,
$lookup with orders collection,
let, define variable customer_id that is from main collection, to access this reference variable inside pipeline using $$ like $$customer_id,
pipeline can add pipeline stages same as we do in root level pipeline
$expr whenever we match internal fields it requires expression match condition, so $$customer_id is parent collection field that declared in let and $customer_id is child collection's/current collection's field
$lookup with orderitems collection
db.customers.aggregate([
{
$lookup: {
from: "orders",
let: { customer_id: "$customer_id" },
pipeline: [
{ $match: { $expr: { $eq: ["$$customer_id", "$customer_id"] } } },
{
$lookup: {
from: "orderitems",
localField: "order_id",
foreignField: "order_id",
as: "items"
}
}
],
as: "orders"
}
}
])
Playground
Tip:
Several joins considered as bad practice in NoSQL, I would suggest if you could add your order items in orders collection as array, you can save one join process for orderitems, see improved version in playground