Add a new field from an object nested inside an array - mongodb

Question

I have two collections:
Assets collection:
{
"_id": { "$oid": "620bc49c937d954ed4acf813" },
"opID": { "$oid": "620bc497937d954ed4acf810" },
"issues": [{
"otherFields": {...}
}],
"__v": 7
}
Operations collection:
{
"_id": { "$oid": "620bc49c937d954ed4acf813" },
"name": "Test",
"otherFields": [...],
"__v": 7
}
I want to add the operation name and _id to all the issue Objects inside the Assets issue field.
My actual pipeline to do it is:
$lookup
{
from: "operations",
localField: "opID",
foreignField: "_id",
as: "operationsJoin",
}
$unwind
{ path: "$issues" }
$addFields
"issues.operationId": "$operationsJoin._id"
$group
{
_id: "$_id",
issues: {
$push: {issues:"$issues"}
}
}
My first issue is at the $addFields stage. The operationsJoin field is an array and when the operationId field it's setted on the issue object it's created as an array as this:
issues: {
operationId: [
"5f3b8ff2906e402014832d33"
]
}
instead of:
issues: {
operationId: "5f3b8ff2906e402014832d33"
}
and i cannot do:
"issues.operationId": "$operationsJoin[0]._id"
And the second issue it's that a doesn't know to add other field to the $group stage apart the issues array that i'm creating.
Anyone has an idea ?? Thanks for the help in advance
Edited:
I let an example of the pipeline here, for now i only want the operationName and operationId fields to be strings, not arrays:
Example Pipeline