[{_id:ObjectId("5e13642ee88247a6b4f2c0f8"),surname:"cooper",comapany:"sabesto",salary:15748,mapped:[{name:"mark",age:"25",surname:"cooper"},{name:"snow",age:"29",surname:"wyte"}]}]

db.collection.aggregate([{$addFields:{data:{$mergeObjects:["$$ROOT",{$arrayElemAt:[{$filter:{input:"$mapped",cond:{$eq:["$$this.surname","$surname"]}}},0]}]}}},{$replaceRoot:{newRoot:"$data"}},{$project:{"mapped":0}}])