db={"someCollection":[{"_id":ObjectId("5a934e000102030405000000"),"img":"base64_image_data_1","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T10:00:00Z"),"tracklet_id":"tracklet1"},{"_id":ObjectId("5a934e000102030405000001"),"img":"base64_image_data_2","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T10:05:00Z"),"tracklet_id":"tracklet1"},{"_id":ObjectId("5a934e000102030405000002"),"img":"base64_image_data_3","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T11:00:00Z"),"tracklet_id":"tracklet2"},{"_id":ObjectId("5a934e000102030405000003"),"img":"base64_image_data_4","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T11:05:00Z"),"tracklet_id":"tracklet2"},{"_id":ObjectId("5a934e000102030405000004"),"img":"base64_image_data_5","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T12:00:00Z"),"tracklet_id":"tracklet3"},{"_id":ObjectId("5a934e000102030405000005"),"img":"base64_image_data_6","taskId":"qy7u17-xunwqu","timestamp":ISODate("2023-09-30T12:05:00Z"),"tracklet_id":"tracklet3"}]}

db.someCollection.aggregate([{$match:{taskId:"qy7u17-xunwqu"}},{$group:{_id:"$tracklet_id",count:{$sum:1},timestamp:{$max:"$timestamp"}}},{$project:{_id:0,trackletId:"$_id",timestamp:1,count:1}},{$sort:{timestamp:-1}},{$limit:20},{$lookup:{from:"someCollection",let:{trackletId:"$trackletId"},pipeline:[{$match:{$expr:{$eq:["$tracklet_id","$$trackletId"]}}},{$limit:1},{$project:{_id:0,img:1}}],as:"image"}},{$addFields:{image:{$arrayElemAt:["$image.img",0]}}}])