db={persons:[{_id:ObjectId("64103dd44c7b1355e4812e87"),sport:"Football",name:"Brian",teams:[ObjectId("62f63b38e6a6cbb9a58c58c4"),ObjectId("64219e4eb5924e070a23f846")]},{_id:ObjectId("642ad97d0c945ff64f2a5f4c"),sport:"Football",name:"Marc",teams:[ObjectId("64219e4eb5924e070a23f846")]},{_id:ObjectId("64269d6c38364df7ca902d75"),sport:"Basketball",name:"Francis",teams:[ObjectId("641453c9adc10762db6997cb"),ObjectId("64269ebc38364df7ca902e17")]}],teams:[{_id:ObjectId("62f63b38e6a6cbb9a58c58c4"),sport:"Football",name:"Team A",persons:[ObjectId("642ada46f785be5eb0042f63")]},{_id:ObjectId("64219e4eb5924e070a23f846"),sport:"Football",name:"Team B",persons:[]},{_id:ObjectId("62f3a57bcc852536a665b179"),sport:"Football",name:"Team C",persons:[]},{_id:ObjectId("641453c9adc10762db6997cb"),sport:"Basketball",name:"Team D",persons:[ObjectId("642ada848b8c7b48102c3cb4")]},{_id:ObjectId("64269ebc38364df7ca902e17"),sport:"Basketball",name:"Team E",persons:[ObjectId("642ada95f8f190e02582e10d")]}]}

db.teams.aggregate([{$lookup:{from:"persons",localField:"_id",foreignField:"teams",pipeline:[{$project:{_id:1}}],as:"moreP"}},{$project:{persons:{$concatArrays:["$persons",{$map:{input:"$moreP",in:"$$this._id"}}]},name:1,sport:1}},{$merge:{into:"teams"}}])