Database
[{_id:0,account:[{accountId:"1234",commonId:"0"},{accountId:"1235",commonId:"1"},{accountId:"1236",commonId:"2"}],sessions:[{sessionId:"6781",commonId:"0"},{sessionId:"6782",commonId:"1"},{sessionId:"6783",commonId:"2"}]}]
Query
db.collection.aggregate([{$project:{merged:{$map:{input:"$account",in:{accountId:"$$this.accountId",commonId:"$$this.commonId",sessionId:{$arrayElemAt:["$sessions.sessionId",{$indexOfArray:["$sessions.commonId","$$this.commonId"]}]}}}}}}])