[{_id:1,users:["a","b"],content:"content stuff",fromUser:"from user stuff",toUser:"to user stuff",messageRead:true,createdAt:{$date:"2022-06-01"}},{_id:2,users:["x","y"],content:"content stuff",fromUser:"from user stuff",toUser:"to user stuff",messageRead:true,createdAt:{$date:"2022-02-01"}},{_id:3,users:["a","b"],content:"content stuff",fromUser:"from user stuff",toUser:"to user stuff",messageRead:false,createdAt:{$date:"2022-01-01"}}]

db.collection.aggregate([{$match:{users:"a"}},{$sort:{createdAt:-1}},{$limit:1}])