db={"conversations":[{members:["124578987898787845658574","124578986532124578986532"]}],"users":[{"_id":ObjectId("124578987898787845658574"),"name":"foo"},{"_id":ObjectId("124578986532124578986532"),"name":"bar"}]}

db.conversations.aggregate([{$unwind:"$members"},{"$lookup":{"from":"users","as":"membersFlat","let":{memberObjectId:{"$toObjectId":"$members"}},pipeline:[{$match:{$expr:{$eq:["$$memberObjectId","$_id"]}}}]}},{$group:{_id:null,members:{$push:{"_id":{$first:"$membersFlat._id"},"name":{$first:"$membersFlat.name"}}}}},{$project:{_id:0}}])