[{/** room data*/_id:"20ae0225-512a-405b-8b9f-d6ffdca6634c",games:[{_id:"cb01da11-5809-43e6-b02e-e878b38f4e11",players:[{user_id:"ef0d7656-38a1-4c82-982b-b4beb3941e07",user_name:"Andrew"},{user_id:"a13de96d-e137-41d8-bd5c-083d9dbc00d7",user_name:"Jack"},]},{_id:"9b03d0ef-178e-49d1-8b86-6b63c1957d6f",players:[{user_id:"ef0d7656-38a1-4c82-982b-b4beb3941e07",user_name:"Andrew"},{user_id:"67acb8b7-1670-4c07-979a-8a6e481bfd95",user_name:"Thomas"}]},{_id:"733a30d5-a6c1-4ce2-a33b-c8dbf61f2a2e",players:[{user_id:"a13de96d-e137-41d8-bd5c-083d9dbc00d7",user_name:"Jack"},{user_id:"67acb8b7-1670-4c07-979a-8a6e481bfd95",user_name:"Thomas"}]}]},{/** room data*/_id:"20ae0225-512a-405b-8b9f-d6ffdca66348",games:[{_id:"cb01da11-5809-43e6-b02e-e878b38f4e11",players:[{user_id:"67acb8b7-1670-4c07-979a-8a6e481bfd95",user_name:"Thomas"},{user_id:"a13de96d-e137-41d8-bd5c-083d9dbc00d7",user_name:"Jack"},]},{_id:"9b03d0ef-178e-49d1-8b86-6b63c1957d6f",players:[{user_id:"ef0d7656-38a1-4c82-982b-b4beb3941e07",user_name:"Andrew"},{user_id:"67acb8b7-1670-4c07-979a-8a6e481bfd95",user_name:"Thomas"}]},{_id:"733a30d5-a6c1-4ce2-a33b-c8dbf61f2a2e",players:[{user_id:"a13de96d-e137-41d8-bd5c-083d9dbc00d7",user_name:"Jack"},{user_id:"67acb8b7-1670-4c07-979a-8a6e481bfd95",user_name:"Thomas"}]}]}]

db.collection.aggregate({$unwind:"$games"},{$unwind:"$games.players"},{$group:{_id:"$games.players.user_name",count:{$sum:1}}})