[{"collection":"players","count":100,"content":{"_id":{"type":"autoincrement","autoType":"int"},"firstName":{"type":"faker","method":"FirstName"},"lastName":{"type":"faker","method":"LastName"},"email":{"type":"faker","method":"Email"},"owner_id":{"type":"int","min":0,"max":15},"mentor_id":{"type":"int","min":0,"max":15}},"indexes":[{"name":"owner_1","key":{"owner_id":1}},{"name":"mentor_1","key":{"mentor_id":1}}]}]

db.players.aggregate([{"$project":{"_id":0,"ownersMentors":{"$setUnion":[["$owner_id"],["$mentor_id"]]}}},{"$unwind":"$ownersMentors"},{"$group":{"_id":"$ownersMentors","playerCount":{"$sum":1}}},{"$project":{"_id":0,"user_id":"$_id","playerCount":1}}])