[{id:"1",title:"First Title",des:"This is desc",contents:[{id:"1",title:"content-1 title",des:"This is desc"},{id:"2",title:"Content-2 title",des:"This is desc"}]},{id:"2",title:"Second Title is Here",des:"This is desc",contents:[{id:"1",title:"content-1 title",des:"This is desc"},{id:"2",title:"Content-2 title",des:"This is desc"}]}]

db.collection.aggregate([{$set:{contents:{$concatArrays:[[{id:"$id",title:"$title",des:"$des"}],"$contents"]}}},{$unwind:"$contents"},{$replaceWith:"$contents"}])