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

db.conversations.aggregate([{"$lookup":{"from":"users","localField":"members","foreignField":"_id","as":"members"}},{$project:{_id:0,members:1}}])