[{_id:0,account:[{accountId:"1234",commonId:"0"},{accountId:"1235",commonId:"1"},{accountId:"1236",commonId:"2"},{accountId:"666",commonId:"3"}],sessions:[{sessionId:"6781",commonId:"0"},{sessionId:"6782",commonId:"1"},{sessionId:"6783",commonId:"2"},{sessionId:"777",commonId:"4"}]}]

db.collection.aggregate([{$project:{merged:{$map:{input:"$account",as:"acc",in:{$mergeObjects:["$$acc",{$arrayElemAt:[{$filter:{input:"$sessions",as:"ses",cond:{$eq:["$$ses.commonId","$$acc.commonId"]}}},0]}]}}}}}])