Mongodb lookup aggregation not getting all field values - node.js

Question

I have two collections which are of schema like
driver
_id:5f9c1d897ea5e246945cd73a
agent_name:"Ratnabh Kumar Rai"
agent_email:"ra2614#gmail.com"
agent_phone:"70****63331"
and
reviews
_id:5f9d54cb3a3ee10c6829c0a4
order_id:"5f9d40f096e4a506e8684aba"
user_id:"5f9bcb66f7a5bf4be0ad9973"
driver_id:"5f9c1d897ea5e246945cd73a"
rating:3
text:""
so i want to calculate the avg driver rating . I tried to use lookup aggregation so that i can get all details and then later calculate the sum...what i did was
let doc = await db
.collection("drivers")
.aggregate([
{
$project: {
_id: {
$toString: "$_id",
},
},
},
{
$lookup: {
from: "reviews",
localField: "_id",
foreignField: "driver_id",
as: "driver_info",
},
},
{
$project: {
agent_email: 1,
orderReview: "$driver_info",
},
},
])
.toArray();
But i am getting result as
{
_id: '5f9d63eb8737e82fbc193dd9',
orderReview: [ [Object], [Object], [Object], [Object], [Object] ]
}
which is partially correct as i also need to get details from my localfield collection that is drivers field, as of now you can see i am only getting id of driver in my projection i also did "agent_email:1" but not getting email

Answer 1

You're actually only projecting _id in the first pipeline and hence only _id is passed to further pipelines, If you need email in further pipelines you need to project it too
let doc = await db.collection("drivers").aggregate([
{
$project: {
_id: {
$toString: "$_id",
},
agent_email: "$agent_email"
},
},
{
$lookup: {
from: "reviews",
localField: "_id",
foreignField: "driver_id",
as: "driver_info",
},
},
{
$project: {
agent_email: 1,
orderReview: "$driver_info",
},
},
])
MongoDB PlayGround : https://mongoplayground.net/p/h7D-tYJ7sLU
[Update]
I realized that you're doing this for getting average and if you need it to be done in a single aggregate query, here it is how you can do it.
Using unwind operator you can flat the reviews array as objects and then group by _id and use the $avg aggregation operator
db.collection("drivers").aggregate([
{
$project: {
_id: {
$toString: "$_id",
},
agent_email: "$agent_email"
},
},
{
$lookup: {
from: "reviews",
localField: "_id",
foreignField: "driver_id",
as: "driver_info",
},
},
// Makes the driver info flat with other information
{
"$unwind": "$driver_info"
},
{
// Now you can group them
$group: {
_id: "$_id",
// Calculates avg on rating field
avg: {
"$avg": "$driver_info.rating"
},
// To make email field appear in next pipeline.
// You can do it for other fields too like Name, etc
agent_email: {
$first: "$agent_email"
}
}
},
{
$project: {
// select the fields you want to display
agent_email: 1,
avg: 1
},
},
])
MonogoDb playground Link