db={"people":[{"id":1,"name":"Pete","occupation":"baker"},{"id":2,"name":"Mike","occupation":"painter"}],"activity":[{"occupation":"baker","activity":"bakes"},{"occupation":"painter","activity":"paints"}]}

db.people.aggregate([{"$lookup":{"from":"activity","localField":"occupation","foreignField":"occupation","as":"activity"}},{"$set":{"activity":{"$first":"$activity.activity"}}},{"$unset":"_id"}])