[{"title":"testone","docs":[{"fileId":"123","fileName":"helloworld.txt"},{"fileId":"124","fileName":"thisisalongtext.docx",}],},{"title":"testtwo","docs":[{"fileId":"345","fileName":"helloworld.txt"},{"fileId":"456","fileName":"thisisalongtext.docx",}],}]

db.collection.aggregate([{$project:{title:1,docs:"$docs.fileId",_id:"$$REMOVE"}}])