Using $lookup and $group to aggregate data - node.js

Question

I am aggregating a large data where i need to group the data according to their types and also i need to lookup the data from another collections.inside $group i want my lookup's data.
my code for aggregation goes like :
NotificationSchema.aggregate([{
$match: condition
}, {
$group: {
_id: "$type",
details: {
$push: "$$ROOT"
},
count: {
$sum: 1
}
}
}, {
$sort: {
_id: -1
}
}, {
$lookup: {
from: "vehicles",
localField: "details.device_id",
foreignField: "device_id",
as: "vehicle"
}
}], function(err, result) {
if (err) {
res.status(500);
return res.json({
result: err
});
}
console.log('res', result[0].details[0]);
res.json({
result: result
});
});
if i remove or comment the $group code i get the data with Vehicle array but using $group i get vehicle array empty, as i have only two types in records in the database, i get two empty array of vehicles. but i have 102 records so i need 102 arrays of vehicles how can i get such result.
what i am getting in console right now is
res [ { _id: 'Vehicle Delay Alert!',
details:
[ [Object],
....57 object...
[Object] ],
count: 57,
vehicle: [] },
and inside every object i dont find vehicle array so i wish to remove vehicle array from here and get a vehicle array that is generated from $lookup inside every object.
Any suggestions are highly appreciated.

Answer 1

You $lookup from details.device_id which comes from an array. To $lookup from a regular field, you can place $lookup after the $match :
NotificationSchema.aggregate([{
$match: condition
}, {
$lookup: {
from: "vehicles",
localField: "device_id",
foreignField: "device_id",
as: "vehicle"
}
}, {
$group: {
_id: "$type",
details: {
$push: "$$ROOT"
},
count: {
$sum: 1
}
}
}, {
$sort: {
_id: -1
}
}])