MongoDB query $group is returning null - mongodb

Question

I am learning MongoDb query and my requirement is to calculate the average time between two dates. I wrote a mongoDB query with project and group stages.
{
project: {
OrderObject:1,
date:1
}
},
{
group: {
objectId: '$OrderObject.pharmacy.companyName',
count: {
$sum: 1
},
duration: {
$avg: {
$abs: {
$divide: [
{
$subtract: [
{
$arrayElemAt: [
'$date',
0
]
},
{
$arrayElemAt: [
'$date',
1
]
}
]
},
60000
]
}
}
},
OrderIDs: {
$addToSet: '$OrderObject.orderID'
},
pharmacyName: {
$addToSet: '$OrderObject.pharmacy.companyName'
},
}
}
The output I get is
{
count: 3,
duration: 54.53004444444445,
OrderIDs: [ 'ABCDE', 'EWQSE', 'ERTRE' ],
pharmacyName: [ 'pharmacy business Name' ],
objectId: null
},
Can someone please tell me why objectId is null in this case but the value is printed in pharmacyName field. I am using this pipeline in parse server as query.aggregate(pipeline, {useMasterKey:true})
The my expectation is pharmacyName === objectId

Answer 1

Most probably your nested element here is with different name:
OrderObject.name.companyName
but this is not an issue for the $group stage since it make the aggregation for all elements( in total 3) in the collection when the _id is null and do not give you any errors ...
It is also interesing why in your output the "pharmacyName" appear simply as "name" ? ;)