Database
[{"_id":"61ef7e5497c9912ac1e56404","name":"Mark","age":20,"isMinor":true},{"_id":"61ef7e5497c9912ac1e56405","name":"Sam","age":22,"isMinor":false}]
Query
db.collection.aggregate([{$addFields:{isMinor:{$cond:[{$gt:["$age",20]},true,false,]}}},{$group:{_id:null,users:{$push:{k:"$_id",v:"$$ROOT"}}}},{"$replaceWith":{$arrayToObject:"$users"}}])