{"organizations":[{"_id":"1","name":"foo","users":{"1":"admin","2":"member"}},{"_id":"2","name":"bar","users":{"1":"admin"}}],"users":[{"_id":"1","name":"john1 smith"},{"_id":"2","name":"bob johnson"}]}

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