Database
[{name:"David",checkins:[{_id:"612e162d439cb04934d13f9e",in_at:"2021-09-12T08:02:00.000Z",out_at:"2021-09-12T09:03:00.000Z"},{_id:"612e162d439cb04934d13f9f",in_at:"2021-09-12T10:04:00.000Z",out_at:"2021-09-12T11:05:00.000Z"},{_id:"612e162d439cb04934d13f9g",in_at:"2021-09-12T12:02:00.000Z",out_at:"2021-09-12T14:03:00.000Z"}]},{name:"Wilson",checkins:[{_id:"612e162d439cb04934d13f9e",in_at:"2021-09-12T08:02:00.000Z",out_at:"2021-09-12T09:03:00.000Z"},{_id:"612e162d439cb04934d13f9f",in_at:"2021-09-12T11:04:00.000Z",out_at:"2021-09-12T12:05:00.000Z"},{_id:"612e162d439cb04934d13f9g",in_at:"2021-09-12T13:02:00.000Z",out_at:"2021-09-12T14:03:00.000Z"}]}]
Query
db.collection.aggregate([{$addFields:{checkin:{$mergeObjects:[{$first:"$checkins"},{out_at:{$last:"$checkins.out_at"}}]}}}])