[{sport:"Football",site:"Bet365",data:[{_id:"1",rating:98.12,bets:[{exchange:"Smarkets",odds1:3.2,odds2:3.3},{exchange:"Matchbook",odds1:1.5,odds2:1.52},{exchange:"Betfair",odds1:2.0,odds2:2.1}]},{_id:"2",rating:99.50,bets:[{exchange:"Smarkets",odds1:2.1,odds2:2.12},{exchange:"Smarkets",odds1:1.4,odds2:1.4},{exchange:"Smarkets",odds1:2.0,odds2:2.05},]}]}]

db.collection.find({"data.bets.exchange":"Smarkets"},{data:{$filter:{input:"$data",cond:{$eq:["$$this.bets.exchange",{$filter:{input:"$$this.bets.exchange",cond:{$eq:["Smarkets","$$this"]}}}]}}}})