db={"workers":[{"_id":ObjectId("1224ec342c2d7202b3ad2af3"),"id_worker":180,"id_boss":100,"hired":ISODate("2005-02-20T00:00:00Z"),"id_group":10}],"groups":[{"_id":ObjectId("6224ebe12c2d7202b9ad9af1"),"id_group":10,"name":"Administrators","addres":"Example"}]}

db.workers.aggregate([{"$lookup":{"from":"groups","localField":"id_group","foreignField":"id_group","as":"group"}},{$project:{"id_worker":1,"id_boss":1,"hired":1,"id_group":{$first:"$group.name"}}}])