[{name:"tester1",reports:[{name:"report1",type:"overflow"},{name:"report2",type:"invalid form"}]},{name:"tester2",reports:[{name:"report3",type:"crossed"},{name:"report4",type:"unknown issue"}]},{name:"tester3",reports:[{name:"report4",type:"try again"},{name:"report6",type:"invalid data"}]}]

db.collection.aggregate([{$unwind:"$reports"},{$replaceRoot:{newRoot:"$reports"}},{$limit:5}])