Deep linking for 3 levels in MongoDB - node.js

Question

I need to retrieve the entire single object hierarchy from the database as a JSON. I decided to use MongoDB with its $lookup support.
So I have three collections:
1. Restaurants
{
"_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"name" : "KFC",
"city" : "Lahore",
"area" : "Johar town",
"min_order" : "500",
"del_time" : "1 hour",
"del_fees" : "5",
"pre_order" : "no",
"rating_star" : "0",
"rating_no" : 0,
"status" : "1",
"working_hours" : {
"monday" : "2pm 6am",
"tuesday" : "6am 8pm",
"wednesday" : "9pm 5pm",
"thursday" : "3pm 6pm",
"friday" : "1pm 9pm",
"saturday" : "4pm 5pm",
"sunday" : "4pm 8pm"
},
"cuisines" : [
"fast food, pizza"
],
"payments" : [
"cash,credit card,paypal"
]
}
2. categories
"categories" : [
{
"_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"name" : "Breakfast",
"category_id" : "1",
"__v" : 0
},
{
"_id" : ObjectId("57c6a06021c8b8d04443aa7f"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"name" : "Special",
"category_id" : "2",
"__v" : 0
}
]
3. items
"items" : [
{
"_id" : ObjectId("57c6da8cc8d053e8310dfe65"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"category_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"name" : "Water",
"price" : "55",
"rating" : "0",
"__v" : 0
},
{
"_id" : ObjectId("57c6da96c8d053e8310dfe66"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"category_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"name" : "Milk",
"price" : "55",
"rating" : "0",
"__v" : 0
}
]
I want an output in Json response like this
{
"_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"name" : "KFC",
"city" : "Lahore",
"area" : "Johar town",
"min_order" : "500",
"del_time" : "1 hour",
"del_fees" : "5",
"pre_order" : "no",
"rating_star" : "0",
"rating_no" : 0,
"status" : "1",
"working_hours" : {
"monday" : "2pm 6am",
"tuesday" : "6am 8pm",
"wednesday" : "9pm 5pm",
"thursday" : "3pm 6pm",
"friday" : "1pm 9pm",
"saturday" : "4pm 5pm",
"sunday" : "4pm 8pm"
},
"cuisines" : [
"fast food, pizza"
],
"payments" : [
"cash,credit card,paypal"
],
"__v" : 0,
"categories" : [{
"_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"name" : "Breakfast",
"category_id" : "1",
"__v" : 0,
"items" : [
{
"_id" : ObjectId("57c6da8cc8d053e8310dfe65"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"category_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"name" : "Water",
"price" : "55",
"rating" : "0",
"__v" : 0
},
{
"_id" : ObjectId("57c6da96c8d053e8310dfe66"),
"restaurant_id" : ObjectId("57c6a02021c8b8d04443aa7c"),
"category_id" : ObjectId("57c6a05421c8b8d04443aa7e"),
"name" : "Milk",
"price" : "55",
"rating" : "0",
"__v" : 0
}
]
}]
}
Still, I'm doing like this
db.restaurants.aggregate([{
$lookup: {
from: "categories",
localField: "_id",
foreignField: "restaurant_id",
as: "categories"
}
}])
It's working fine for Restaurants and categories but how I do it for each categories item that I have in categories?

Answer 1

You could try the following aggregation pipeline, bearing in mind that you'd need to do an $unwind operation on the created lookup array in order for the next $lookup operation to apply:
db.restaurants.aggregate([
{
"$lookup": {
"from": "categories",
"localField": "_id",
"foreignField": "restaurant_id",
"as": "categories"
}
},
{ "$unwind": "$categories" },
{
"$lookup": {
"from": "items",
"localField": "categories._id",
"foreignField": "category_id",
"as": "categories.items"
}
}
])

Answer 2

I don't think that's directly possible. Per the documentation on $lookup:
If your localField is an array and you’d like to match the elements
inside it against a foreignField which is a single element, you’ll
need to $unwind the array as one stage of the aggregation pipeline.
So, basically you need to $unwind your categories array in order to look up items. But then you can get creative as far as piecing it back together.
For example, this gets close to what you want:
db.restaurants.aggregate([
{
$lookup: {
from: "categories",
localField: "_id",
foreignField: "restaurant_id",
as: "categories"
}
},
{
$unwind: "$categories"
},
{
$lookup: {
from: "items",
localField: "categories._id",
foreignField: "category_id",
as: "items"
}
},
{
$project:{
category: {
elem: "$categories",
items: "$items"
}
}
},
{
$group: {
_id: "$_id",
categories: {$addToSet: "$category"}
}
},
{
$lookup: {
from: "restaurants",
localField: "_id",
foreignField: "_id",
as: "restaurant"
}
},
{
$unwind: "$restaurant"
},
{
$project: {
_id:false,
restaurant_id:"$_id",
categories:true,
restaurant_data:"$restaurant"
}
}
]);