[{_id:"1",testId:"121",testName:"test1",result:"pass"},{_id:"2",testId:"122",testName:"test2",result:"pass"},{_id:"3",testId:"123",testName:"test3",result:"pass"},{_id:"4",testId:"121",testName:"test1",result:"fail"},{_id:"5",testId:"121",testName:"test1",result:"fail"},{_id:"6",testId:"122",testName:"test2",result:"pass"},{_id:"7",testId:"123",testName:"test3",result:"fail"},]

db.collection.aggregate([{$match:{"testId":{"$exists":true}}},{$sort:{_id:-1}},{$group:{_id:"$testId",testDetails:{$push:{result:"$result",testName:"$testName"}}}},{$addFields:{testDetails:{$slice:["$testDetails",30]}}}])