[{"_id":{"$oid":"5ff09030cd55d6d9f378d460"},"username":"a value","userid":"123456","last_access_ts":1612426253,"last_access":"2021-2-4 9:10:53","anotherid":12345678910,"verified_date":"2021-1-2 16:24:32","verified_ts":1609601072,"group_users":{"-1001159747589":[{"anotherid":12345678910,"userid":"123456"}],"-1001143137644":[{"anotherid":12345678910,"userid":"123456"}],"-1001368608972":[{"anotherid":12345678910,"userid":"123456"}]},"registered_access":"2021-1-2 16:24:42",}]

db.collection.aggregate([{$match:{username:"a value"}},{$project:{_id:1,userid:1,groups:{$objectToArray:"$group_users"}}},{$unwind:"$groups"},{$lookup:{from:"collection",let:{anotherid:"$groups.v.anotherid"},pipeline:[{$match:{$expr:{$in:["$anotherid","$$anotherid"]}}},{$project:{_id:0,userid:1,username:1}}],as:"groups.v"}},{$group:{_id:"$_id",groups:{$push:"$groups"},userid:{$first:"$userid"}}},{$addFields:{groups:{$arrayToObject:"$groups"}}}])