[{"rounds":{"matches":[{"id":"8cac353e-fb04-4a9a-b340-ed64e1094ff1","confrontationId":"2f2c58c9-f1be-4914-8aee-03d00725a0be","arena":"Tacoma Dome","status":"FINISHED"},{"id":"d8ff6322-824c-455c-b59d-d837d0188d93","confrontationId":"41d85805-5571-42c6-a38e-e03fd8fcdc96","arena":"Baku Crystal Hall","status":"FINISHED"},{"id":"e23dc4ce-24de-4f7f-b5b0-9b124ec0e71a","confrontationId":"2f2c58c9-f1be-4914-8aee-03d00725a0be","arena":"Greensboro Coliseum","status":"FINISHED"}]}}]

db.collection.aggregate([{"$unwind":"$rounds.matches"},{"$group":{"_id":"$rounds.matches.confrontationId","items":{$push:"$$ROOT"}}}])