How do I compare two nested documents using mongo_dart - dart

Question

Here is my database entry structure, it has a nested document called friends, I want to compare two different _id's friends list in dart using mongo_dart
{
"_id" : ObjectId("60ae06074e162995281b4666"),
"email" : "one#one.com",
"emailverified" : false,
"username" : "one#one.com",
"displayName" : "complete n00b",
"phonenumber" : "",
"dob" : "",
"points" : 0,
"friends" : [
{
"username" : "three#one.com",
"sent" : ISODate("2021-05-26T10:01:30.616Z")
},
{
"username" : "six#one.com",
"sent" : ISODate("2021-05-26T10:43:16.822Z")
}
]
}
Here is my code, but I am not getting any returns
Future<Map> commonFriends(store, myObjectId, theirObjectId) async {
var commonList = await store.aggregate([
{
'\$project': {
'friends': 1,
'commonToBoth': {
'\$setIntersection': [
{'_id': myObjectId, 'friends': '\$username'},
{'_id': theirObjectId, 'friends': '\$username'}
]
},
}
}
]);
return commonList;
}
I am getting an error from db.dart which is apart of mongo_dart package. The error is "Exception has occurred.
Map (4 items)"