Get aggregate with lookup in single Object - node.js

Question

my problem is how I get a simple object of ownerDetails, correct me if my code is wrong
data I got, from my code
{
"status": true,
"data": [
{
"_id": "61a2a9680b122bc154cbd6af",
"ownerName": "Ajay",
"mobile": 878787878,
"designation": "IT",
"gender": "Male",
"age": 26,
"carDetails": [
{
"carName": "BUY",
"color": "blue",
"sheets": 8,
"avgSpeed": 105,
"price": 850000,
"model": "C110"
},
{
"carName": "GTR",
"color": "blue",
"sheets": 4,
"avgSpeed": 105,
"price": 98000,
"model": "G120"
}
]
}
]
}
i want data like this,
{
"status": true,
"ownerDetails": {
"_id": "61a2a9680b122bc154cbd6af",
"ownerName": "Ajay",
"mobile": 878787878,
"designation": "IT",
"gender": "Male",
"age": 26,
"total_car": 2,
}
}
code for getting data from collections
exports.getOwnerDetails = async (req, res, next) => {
try {
Owner.aggregate([
{
$match: { ownerName: req.body.ownerName }
},
{
$lookup: {
from: "cars",
localField: "ownerName",
foreignField: "ownerName",
as: "carDetails",
},
},
{
$project: {
"carDetails._id": 0,
"carDetails.ownerName": 0,
},
},
]).then((data) => {
res.json({
status: true,
data: data,
});
});
} catch (error) {
res.status(500).json({
status: false,
msg: "Error : " + error,
});
}
};
in may returning data, I got an array of carDetails, but I only need that how many are owned by an Owner, and return a simple object of ownerDetails

Answer 1

$lookup return an array because more than one values can match the join. If you want to get only one value you can get the first element from the array. Then you can add these aggregation stages:
First $addFields to get the first element from the array (index 0).
Then use $project to get the desired output.
{
"$addFields": {
"data": {
"$arrayElemAt": ["$data",0]
}
}
},
{
"$project": {
"status": 1,
"ownerDetails": {
"_id": "$data._id",
"ownerName": "$data.ownerName",
"mobile": "$data.mobile",
"designation": "$data.designation",
"gender": "$data.gender",
"age": "$data.age",
"total_car": {
"$size": "$data.carDetails"
}
}
}
}
Example here

Answer 2

Query
i am not sure if this is what you want
unwind all owners from data
and from each owner, count its cars number
Test code here
aggregate(
[{"$unwind": {"path": "$data"}},
{"$set":
{"data.total_car": {"$size": "$data.carDetails"},
"data.carDetails": "$$REMOVE",
"_id": "$$REMOVE"}},
{"$set": {"ownerDetails": "$data", "data": "$$REMOVE"}}])